How will you know that you are infected from anal sex?

You wont unless you ask him.

Ask. There are no ways for you to find out on your own. Just ask. If she won't tell you, you just have to deal with not knowing.

Here you will never come to know, as there is no heymen in the anus you will have to believe her.

Here it is not possible to tell as there is no hyemen to break. But if she has had anal sex often in the past you can tell by the size of her anus hole.

There is actually no physical way to tell if a woman has had sex before (unless she's pregnant).