The Lost Lands relative tech levels are divided among the following:
High Middle Ages
Age of Sail
Starting with Cults of the Sundered Kingdoms (currently at the printer), each book that reveals gazetteer-type information about the Lost Lands will include at the beginning a list of the tech levels and a description of what each one means to the game world. Plus each region listing throughout the book will include a line that gives the tech levels.
In general the Lost Lands runs around a High Middle Ages - Medieval tech level, trending more towards Medieval in the cities and HMA in the countryside. The greatest cities are sometimes Renaissance. Some specific areas (Razor Coast, Oceanus, and Southvale to name a few) are Age of Sail. The Blight (the City-State of Castorhage) is more or less the only major area of Industrial Revolution tech, but as you'll see, it's a tech more driven on magic than anything else. Sort of a steampunk without the steam and with necromancy instead.
The relative tech levels are maintained primarily by the level of isolation of an area and general need.
Only the most isolated and backward areas would have a Stone Age or Bronze Age tech level. Exceptions would be:
Large parts of Libynos, where the vast quantities of cheap slave labor, extremely strong dynastic rule, and a desire of the powers that be to prevent equipping the peasantry with the means to effectively revolt keeps things at a Bronze Age level (think Khemit). That is not to say that no tech above BA exists in Khemit, just that the rulers keep a lid on trade in that stuff to the degree that unless someone has a great deal of money, they're not going to be able to easily acquire stuff of a higher level. The powerful elite would have access to a higher tech level, and adventurers conceivably could with their usual abundance of loot, but the average Joe on the street is basically stuck with BA tech.
And a few anachronistic specific areas that keep it at that level for their own idiosyncratic reasons, like the city of Freegate. Freegate is one of the major ports controlled by Bard's Gate but is so strongly steeped in its ancient Hyperborean roots basically maintains a BA tech level in their day-to-day existence. They've got all kinds of higher tech coming through to and from Bard's Gate (which is Medieval, borderline Renaissance), but they have simply chosen to not embrace it--something they have the luxury to do as a protectorate of Bard's Gate. Since Bard's Gate will handle any major military or trade-related problems they might face, the aristocracy of Freegate just kicks back in their togas and debates philosophy at the Forums while the coin rolls in. Visitors to Freegate are almost always of a higher tech level, but (as with the Libynos example above) the tech level indicates the most prevalent tech level, not the tech level available for those with coin enough to buy it or who may be traveling through.
Dark Ages is most frequently seen in not necessarily isolated areas, but areas that are on the fringe of known civilization. Higher tech levels are available but are generally expensive and the folk get along just fine without them for the most part. There is some tech-level creep, but when magic is readily available to take care of a lot of the problems tech was designed for then it reduces a lot of the drive to develop/adopt it. If necessity is the mother of invention, then cheap and plentiful magic is frequently guilty of matricide. Examples of typical Dark Ages tech would be the Northlands, the Heldring Peninsula, the island of Ynys Cymragh.
High Middle Ages tech is pretty much the default for most of the known civilized lands. The typical country village or small city is likely to be HMA/
Medieval is the highest "normal" tech levels. Most of the more powerful and/or affluent cities are going to be here, with the tech level descending more towards HMA in the hinterlands as you get farther away from the population centers.
Renaissance are the truly outstanding cities of the world. No nation is entirely renaissance. This represents the pinnacle for most technological achievement in the Lost Lands. Bard's Gate probably bounces right at the cusp between this and Medieval, but could probably properly be called Renaissance (I'll make a final decision when the new Bard's Gate manuscript arrives from Casey Cristofferson and I can see what he added on top of the old stuff). Other than Bard's Gate, the most likely places to find Ren tech is in Courghais, the imperial capital of the Kingdoms of Foere (though Medi and HMA throughout the kingdoms themselves), a few of its more affluent sister cities, Reme, some of the colony/conquered cities Oceanus like Oestre on the Matagost Peninsula), and maybe Alcaldar (still deciding on that one).
Age of Sail is really only found in nations that principally rely on their naval capabilities to project their power. The Empire of Oceanus is the greatest example of this and it and its colonies explain the perfusion of its tech in remote areas such as the waters and shores around the Razor Sea. There the availability of spellcasters is probably a bit more limited, plus Oceanus's tech is fairly common, plus Oceanus stands to gain a lot of trade by pushing their tech on these outlying areas that have no real hope of ever contesting with Oceanus but will gladly pay hard coin for the tech that they offer, which as a result is probably a lot cheaper than it might be found in other places. The Kingdoms of Foere (easily Oceanus's biggest political rival) would likely be at this level except it's largely become a land-locked empire in the last few centuries since its major naval defeat by Oceanus at Kapichi Point and has, therefore, not had the need to develop this tech. The Foerdewaith remain the major dominant power on the Akados mainland (and to a certain extent in Libynos as well, as seen in the Great Crusades of previous years) and with its tenuous peace with Oceanus chooses not to engage in a maritime arms race. Oceanus knows it can't beat Foere on the mainland, and Foere knows better than to invest in defeating Oceanus at sea. So a status quo exists that does a lot to prevent the proliferation of Age of Sail tech throughout the mainland. Plus Foere has the largest population in Akados and like a gazillion spellcasters, so the need doesn't seem pressing.
Industrial Revolution tech basically wouldn't exist without the City-State of Castorhage (The Blight) and essentially doesn't exist outside its local and colonial bounds. The only nation that could truly contest with Oceanus at sea (though no doubt at great cost), the two maritime powers basically steer clear of each other's assets to avoid any costly entanglements. And, as mentioned, Castorhage is Industrial Revolution in effect only, in that its tech emulates Industrial Revolution norms but relies more on necromancy and other magic to make that happen. The Blight is kind of a weird place. To get a feel for its tech (and weirdness), read Richard Pett's masterful novel: Crooked available here. It is not a Frog God Games product, but our agreement with Richard to make sure he never gets "Styed" again (if you're familiar with those events of the late days of Dungeon Magazine) lets him keep using his Blight IP while FGG creates Lost Lands game products out of it. Rich's novel takes place in the city of "Brine", but you'll easily see that it is basically The Blight in novel form.
We've essentially designed the Lost Lands to be able to seamlessly incorporate all of the old Necromancer Games adventures (Crystal Skull, updated in Cults of the Sundered Kingdoms included firearms) as well as new Frog God Games classics like Nick Logue's Razor Coast here and allow those kinds of play but not force GMs and players who want to play in a different kind of game that doesn't involve firearms and other higher levels of tech. It'll even allow low tech play in Khemit, etc. for those who want it, all without forcing components on them that they don't want to use. A GM can include or ignore Razor Coast in his campaign, include or ignore the fact that the Imperial Guard of Oceanus includes a battalion musketeers, include or ignore the fact that there may be gunslingers wandering around the countryside. Other than those places where they have been specifically called out, firearms and tech will not play an important role in FGG books and adventures and left up to individual GMs to insert if they wish. And if you think the temptation of a firing line of muskets would be too much for any government to resist fielding, then you haven't seen what a few well-placed fireballs can do to a line of stationary guys carrying horns full or blackpowder or what a wildfyre cannon can do against a wooden ship with a magazine full of powder and shot. ;-)