But remember - not all cars with D/A/F/B/E/CH etc. stickers are "illegal" - on some cars people left them on because a) they don't care, it's a pain in the ass to remove the sticker or some similar reason or b) some poeple (some kind of serbian "rednecks") like to have a sticker from the country car came from, because then "neighbourhood can know that I can buy a car from Germanu/Austria/Switzerland etc" ! Like this Citroen XM - it was imported legaly long time ago(2003. I think) and still has "D" sticker on the bumper:

http://www.mojauto.rs/polovni-auto/401192

But anyway - whatewer the case is you can always know from which country the car was imported.