That’s a great question, but there’s no certain way to know, so I’ll just mention a few basic guidelines.

First, of course, is to be aware of the health cues she is giving you. If she says her stomach is bothering her, or anything of that nature, I pretty much assume that anal should be put on the shelf for the time being. All people tend to be sensitive about the state of their bowels and anus, and your girlfriend is no different, if she doesn’t feel clean and healthy back there she’s going to get upset if you start moving to anal play. So, listen to her. It doesn’t hurt to ask her if she feels okay if you are getting any sense that she might be spending more time in the bathroom than usual.

Also, watch how she’s eating. In general, anal does not go that well after large meals, spicy meals, or strange foods. Trying anal the night you took her out to eat spicy Indian food is usually a doomed idea.

Ideally, she should not have eaten for a few hours, and should not have a full stomach.

Frankly, I try to be aware of when my woman has been to the bathroom when I’m feeling like a little anal experimentation. The ideal time for anal is about an hour after she has taken a good shit, so, if you are intimate enough with her to know that, it can help.

Finally, the best test is simply to start playing with her anus a bit during sex - if she makes protesting noises, try to be sensitive to her tone - sometimes a protest is just a “I’m saying no, but I could be convinced” kind of thing, but if she is feeling unclean or in poor stomach health she will have more urgency in her tone of voice.

I like to use the “reach under” as this kind of test the anal reach under (if that link doesn’t work type “reach under” into the search box on the home page)

Ultimately, like a lot of the questions about anal sex, what you need is a kind of consciousness and awareness of your partner’s state of mind and her experience.

And finally, I want to repeat this especially, with surpise anal sex, it’s often best not to take it to far, not to try to put it in deep, and not to do it for more than a few minutes. You want your partner to come away from the experience feeling a little titillated with her own daring and sexiness, and proud that she was able to take you back there, even if it was only an inch or two deep, and only for a minute or three.

I hope that helps.