It looks like someone pumped a load in her and he pulled his cock out and she just pulled her panties up without her even trying to wipe the cum away.
Either that or she masturbated before she went out and let the pussy juices from her masturbated pussy drip into her panties, again without wiping it away.
As far as getting her to tell, while you are fucking her you need to convince her how much it turns you on thinking about her taking someone else's cock. If you talk about how you like it often enough, she might eventually let her guard down and talk about it.
