Maybe someone spilled a white-russian on her...leaving white stains?

You can just go with what she originally told you. You can encourage her to go to another party, then you show up later. You can try to pry any further details from her, but also risk making her mad...or she might give up the details on her own. Ummm, try to get to know someone else that was at the party, and get details from them.
Whatever you do, if you want your wife to cuckold you, or want her to be a hotwife...make sure to be decent about the whole thing. Don't make her mad. Make sure that she knows you are very much ok with her relaxing with another man...but that she not lie to you, and tell you all about it. Above all, make sure she knows how much you lover her...no matter what happens.