I would be shocked if they asked for such evidence, unless they really felt like you were lying. There is alot we put on our applications that adcom's just have to believe. I mean, how do you prove you're a minority or URM? It's based on self-identification. I also have that I came from an underprivileged environment, and I never even considered how I would prove it. Granted it was in the US, and when I listed my home county on AMCAS it automatically came up as "rural" and an "underserved" area. I guess maybe that's some proof. But that doesn't prove that my family was living under poverty income, or that we were on welfare for some of that time. I don't know. I just don't think it would come up. It seems like a very controversial thing to ask you to prove you came from an underprivileged community and lived in low income housing. I wouldn't worry about it too much. Even if you're asked about your upbringing, you'll sound knowledgeable enough, so that should be confirmation that you are telling the truth.