I do this occasionally...wonder if someone is trans. It's only ever if I notice something that gives them away as bio-female but they seem to be presenting as male, however (haven't yet seen someone who seems to be a trans lady that I can recall). I'll admit sometimes I want to ask, but I never do for a number of reasons.
Otherwise I don't really wonder about people. Of course I recognize them as one gender or another, or at the least one sex or another, but it's never for any significant reason: if I can't "figure it out" it doesn't bother me and I just move on. Most I ever wonder is if someone is, in fact, trans, and whether they're out and living as the gender they really are -- part of that is that I have always worked in service industries (primarily retail) so I don't want to use a pronoun that may offend or ruin someone's day/mood (especially as I know what that's like), but I'm sure part of it also is the "are they like me?" sort of thoughts.
I don't think it's biased at all or anything. It's human nature, like others have said.