2) ask the question directly, "Why didn't you edit/register? ... x-1) Thought it was a male-only club, x) Oh, isn't Wikipedia a nerd cabal?, x+1) Because Wikipedia feels like a WASP-only thing" etc. This would require a lot of effort to come up with a good phrasing to cover all "discrimination" feelings and to avoid leading/loaded/biased questions which would skew results, but doesn't sound impossible. (Profs in my university regularly do such things for sexual harassment and other discrimination surveys in order to assess the scale of the problem.)