I would refer to Gambler's methods in this case.
If you may be sitting down with her and talking to a friend you could put your hands down her pants or something.
As Gambler says; this pleases her bad- and good-girl. You actually do something sexual in public but without anybody noticing it.
Other way to do it is in a totally playful way;
"Oh, the material of your jeans feels nice! Is it the same at the back?"
And you touch her ass with a huge smile on your face.
This advice comes from a master, so give it a try
