I think that true attraction would be pretty obvious to notice. Otherwise, there aren't really any reliable signs, from what I've read. More people seem to be bi-curious than actually bisexual. It probably has to do with the difference between emotional attraction (which would be considered bisexual) and physical attraction (which could be bisexual or bi-curious behavior).
The only way to know for sure is to talk with the guy. Chances are good that if he is bi-something, he won't tell you the truth for a long time. There's almost as much stigma attached with being bisexual as being gay (or even more, in the case of men).
That's my guess, anyway.