I actually looked through a few peepholes (they didn't even have glass, they just had doors with holes where a real peephole might go!) for some of the apartments in the building I was renting in. These units weren't even renovated, they were just concrete shells! The other way to tell is to count how many lights are on during your night walk. First thing you find is that units with windows facing into the complex are usually more occupied than the outer ones (makes sense, noise is a big problem in the latter).

And traffic/parking was pretty bad at that complex, if 25% of the units were unoccupied, I couldn't fathom what full occupancy would actually look like.