[Joker_vD]

Well, those are quite close to each other from the orthographic point of view, I guess: Ukrainian or Serbian are visually very distinct from either Russian or Bulgarian, while to tell the latter two apart you need some actual knowledge about the differences of those languages: say, that the abundance of letter "ъ", words ending in "ът"/"та"/"то" and tons of prepositions (i.e., often repeated two-three letter words) are a pretty good indication of a Bulgarian text.