Most girls will say yes. And you need to ask her gently, sexily, something like, "I'd love to eat your ass. I bet it would make you feel spectacular.
Trace a fingertip along the area where their butt cheeks meet their thighs. Another note of caution: if your partner is female-bodied, don't lick.
The secret to eat a girl's ass is that you don't eat her ass, not right away at least. In sex, the journey is often more pleasurable than the destination. So, you start.