[Jeans]

if it happens in the course of running night bar game, just announce to her that "you'll see about that" and calmly put your hand on her ass and squeeze. If her ass is flat call her out on it, but if it really is plump and juicy just say something like "it's agreeable." Girls can be a lot more comfortable with that level of touching than you might think as long as some attraction has been kindled.