The women in my family that are married or dating a white male stated that you can tell easily. I for one can't. I've had conversation with several while waiting in line, but I don't think that's showing interest. I take that as friendly small talk. I've never dated a white man, but have seen several that has caught my interest throughout my life. Please give me some insight on this; I may have came across some that was interested and not even realized it.
My instinct is to say "the same way you tell any man is interested". But I feel like there's a problem of expectations on both sides. Some white men seem to be nervous about approaching black women they are attracted to, because they assume they won't be interested, and it seems to be the same the other way around. My suggestion would be to cut through that and make the first move if you think the guy is interested, hopefully you won't have to put up with any BS if you're wrong.
But if you think they are interested, most likely they are, but they assume you won't be.