I would recommend asking if she has any sexual fantasies/things she'd like to try and if she doesn't mention anal then ask in a jokey way and see what she said. Don't be like 'do you want anal?' But 'would you ever have anal?'. If she says no then sorry, you're not getting any!
I think you can ask this after you're in a steady sexual relationship so after about 2 months unless you get comfortable very quickly