if you know the cars then you know just by looking
for example i have the specific look of each "generation" of honda civics memorised (for '83 and newer). so i can instantly look at one and say something like "that's a '92-'95"
i can tell if it's a '96 LX (in canada) because the body style is the same as the '97-'00, but the wheels are smaller and different looking
and the '99 and '00 have different bumpers than the '96-'98, even tho once again the body shape is the same