I'd say gradually work your way back during cunnilingus. Tease the taint, then go a little further. By that time, she should be pretty relaxed/excited and open to anything, unless she has a major hang-up, then she'll tell you (or at least give you enough body language - do pay attention), at which point, of course, you respect her boundaries. If that goes well, slowly start working a finger in while you are orally servicing her clit. See how she feels about anal penetration by her reaction.
If that has gone well, you can certainly discuss it. Or you can just keep trying new things and watching her reaction. Keep in mind, you need to really have her warmed up (again, I suggest oral) before you move onto anything else. You can also try working a finger in her butt during vaginal sex. Many women like the double penetration.
Later on, trying out a small plug (vibrating is probably better) with lots of lube. Obviously, make sure she knows what's coming before you slowly work it in.
Remember, she may not have done any anal play before, or she may have had a bad experience. The key is to earn her trust by respecting her boundaries and earn her lust by keeping her coming from other, more familiar activity before you start making and backdoor advances.
Now, if you are looking to be the receptive partner on ass-play, I think that usually comes naturally after getting her to enjoy receiving first. You can give her the opportunity to play with you simply by being accessible and providing verbal and non-verbal feedback when she is doing something you like.