The maps I've seen don't display political boundaries. Could you direct me to a map that does show political boundaries?
You should read the basic Forgotten Realms Campaign Guide than.
It has political boundaries, though being a 3e book perhaps that's not the type of book you want to read. It gathers information found in books and modules up to that also included local political boundaries. That said, it is correct and incorrect at the same time.
It is correct for the time it was written, but not absolutely correct for the Forgotten Realms.
The Reason is probably better shown by example than just stating.
The Forgotten Realms has different sovereignties, such as independent city states, Nation States, and Regional areas (such as the Dale lands).
Could you tell me the exact boundaries of Athens, Sparta and Corinth? Anything that tries to define them absolutely normally is wrong because the areas of control varied at different points in their existence as city states. They DID have boundaries normally as cities, at least defined at various points of time, though the actual area they controlled went further.
For these types of boundaries you can see them very easily in the first box set of the Forgotten Realms released with what many call the Gray Box.
I expect that you are not looking at it from a City-State perspective though (which is what the early Forgotten realms were really written in regards to, more of a City-State political control aspect, though there were nations as well, there were a LOT [and still are} of independent cities.
There were Kingdoms set up though, such as the Moonshaes. These are also a region. The Islands were normally shown with pretty distinct borders (as they were Islands).
Now, if you were wanting area of control, that is VASTLY DIFFERENT than political borders. For example, what were the borders of the United States in 1780? It is actually rather nebulous, especially when you consider the areas the French claimed, the Spanish Claimed, the British Claimed, and the Colonies claimed.
Even in the 1820's-1850's the borders of the United States were more nebulous with areas claimed by both the United States and Mexico. This is one reason the Mexican-American War was fought between the United States and Mexico.
This is also the time periods (U.S. Westward expansion, City State period, etc) which most resemble the Realms.
This makes for a rather rich political climate if you want to run one like that because War and Politics go hand in hand with border contentions like that.
You may want something more like the more recent 80 years (because before the 1940s, borders were still highly contested in some areas, even in Europe itself, for example the highly contested region along the border between Franch and Germany which is seen by some as one of the sparks for WWI and WWII) where political boundaries are finally more or less seen as rather stable.
That still doesn't address the idea of control. While one may have regions, what are the actual areas of control?
The United States claims at least areas up to 20 miles from it's borders (normally more easily enforceable on the water than land though, and to avoid contentions, this is where it normally takes place, with some overlap in waters between the US and Mexico and Canada).
China claims control over areas up to 200 miles from it's borders.
So, in that light, if you want a 21st century take on political borders being defined rather than the more traditional take from the past several thousand years, that's rather easy. Just utilize the Forgotten Realms Campaign Setting Guide which was written for 3e. That gathers all the various information from past books and modules into a comprehensive map with what you can call your political boundaries.
If you want more of a take from what it was like previously with defined walls for city states, and descriptions of where the Kingdoms and lands extend to and from (as found in the Grey box) you can use the Campaign Box from 1e.
Both have political boundaries defined, but in different ways depending on whether you want the Frontier is always expanding but undefined, go find and adventure in the wild (1e) to the more modern tastes of everything is more or less civilized, go adventure in the known world (3e).