In general, what we do here in the US is to just assume all competitors at our competitions are USA. If somebody wants to represent a country other than USA, they need to prove citizenship of that country--usually by presenting a passport. Yeah, this does mean that there are perhaps quite a few people representing USA that do not have citizenship, but it would be overly tedious and demand too much time to verify that every competitor at every competition has citizenship. And what if a competitor didn't have proof of citizenship on them? Would we turn them away and not let them compete? Of course not. In general, most people have citizenship of the country in which they are living. And if they don't, they can either keep their mouth shut and claim citizenship anyway or show us that they are indeed a citizen of a different country.