That's a tough one, pickle.

You may not be having R talks and so are still functioning as "roommates" so you still have to operate under that premise. If she is going to date someone, then that is what she is going to do.

I know that you are still hoping that things will change and she will want to be with you again as a partner and possibly wife.

Still, why is she still living in the house? Is it (for her) just a convenience? Money? Something else?

What 180s have you been doing that might have helped her understand that you are a different man and so she might consider a new R with you?

Is her going out with this GF something new for her? Is this the first time she's been out to 3am or done overnights with friends?

There's nothing you can do other than wait and see if there is someone else. If you talk to her about it, she could very well lie and keep it deep underground. If you don't say anything about it, she may open up eventually or let it slip, and you can make any decisions at that time.