I've been reading a lot on the Yugoslavian Crisis (which barely fits into the 20-year range, but is clear by a few years, at least for part of the conflicts), and had a question similar to that of why Jews couldn't blend in and "pass" during the Holocaust. How did the Serbs, Croats, and Bosnians distinguish between one another? There are reports and claims of ethnic cleansing, especially by the Serbians according to the text I'm reading...but how did they figure out who was who? Was it based on language? Did the people of each part of Yugoslavia all get categorized based on what region they lived in (before independence was claimed)? And if so, then how did Serbs distinguish between Bosnian Serbs and just regular Bosnians?
Thanks for the help and clarification in advance!
Although ethnically (in terms of genetic heritage that is) quite similar, Bosniaks, Croats, and Serbs all have distinct national identities. Many of them are proud of their national heritage, as all three have distinct histories, as all three were at one point or another an independent Kingdom. Though the Balkan States have been for much of history controlled by external masters (Hungary, Austria, and the Ottoman Empire come to mind in particular) the three never lost their sense of national identity.
As you pointed out, language is a large distinguishing factor. The three do have separate languages, although they are quite similar to the point that speaking one could be understood by a speaker of another language with decent comprehension. One major difference between the languages however is the alphabet they use. Croatian uses only the Latin alphabet while Serbian and Bosnian use both. And even if you, as a speaker of one of the languages, could understand one of the other languages, you would still obviously be able to tell they were not speaking your language.
To address your last question, Bosnian Serbs are simply Serbs who live in Bosnian. The phrase is similar to African American, though in the case of Bosnian Serb the region they live in comes first and their ethnicity comes second.