Most Helpful Guy
I don't have any experience, but from what I've heard, if you feel that she's being very receptive, not moving away when you get close to her, and is comfortable with you touching her arm, hands, etc and she's being very attentive and/or holding strong eye-contact, and you feel like it's a climax in the interaction, that's a good time to go in for one.
But really, you don't have much to lose. The worst that can happen is that she'll turn away her face and tell you that she's not ready. If she does that, just proceed normally like nothing happened.