If you can get her to recall some of the night's activities (other than the sex part), that might help determine if she was, at least, with a black guy. For example, did the man, she was with, take her out to eat after she supposedly had sex with him? I know this sounds crazy but stay with me a sec ... if he took her out to dinner and she recalls paying for their dinner, then there's a good chance she's telling you the truth; just get HER to volunteer that tid-bit of information.
Black men are the only ones that can regularly get white women to pick up the dinner tab ... I swear! It's like the women they fuck feel obligated, for some reason, to pay for the good fucking they got.
There's a story behind all this ... one I've told before, that comes from many times observing mixed couples at restaurants. Mac