Now that we're all allowed to wear pants, not so much.

Kneeling is not a major sign, though legs apart may be...
What I usually find is that things which seem so BDSM to a person listening for it will go right over a vanilla's head. For example, the "fetch boy" at my job (whom is trying to switch to another department and I really, really hope he gets the position) will reply to every thing I say to him with "Yes, Miss" or "Thank you, Miss", but he doesn't give any other supervisor such titles. Is that a clue as to what he is? By itself I suppose it's rather ambiguous, except combined with a hundred other subtle things that he does all day which wind up with us having this lengthy unspoken conversation that nobody else is aware of that I can't seem to ignore (I swear there is something wrong with me that I continue to be attracted to this person).

Okay, that was a tangent. Back to my point, which was 'is kneeling a sign?' I think it would attract attention if the right person was watching, but to vanillas it's nothing at all. However that alone, by itself, isn't enough to make anyone sure, either. Case in point? Eventually, you have to just plain ask.