I am sorry. I know how it hurts. You may want to wait awhile before you do anything. Just get yourself together before you decide how to handle it.
No, threats don't work.
One thing that I used to do. can't really say that I would necessarily advise this but, I used to ask my wife a question that I already knew the answer to and I knew that she would lie about. When I asked her I would watch closely and observe her body language when she lied so that I would recognize it in the future. That's me, analytic and calculating.
If your style is more straighforward, let's-not-play-games, which is better I think, just tell her that you know that she was there last night. Look at her when you do it so she knows you're not just guessing. Then wait for the nuclear blast. They don't like it when they're caught and remember, in case you've forgotten, it's all your fault anyway.
Lying is disrespect. If you confront her and tell her that you know, then at least she knows that you're not as stupid and blind as she hopes you are. Therefore she has to at least respect you a little, even though she would rather not.
If you confront her, be polite but assertive. You don't have to tell her how you know, that's your business. She will probably want to know how you know and may even suggest that you're having her followed or something. Don't rise to the bait. She wants to know so that she can cover her tracks better in the future.
It also gives you the opportunity to explain your feelings about continued contact. No lovebusting though. Make sure you remember. If you start feeling like you're going to boil over, take a break. And no neediness. You can just explain that it hurts you and prevents marriage from recovering. Then just leave it at that. Don't repeat yourself because she tries to tell you that your wrong to feel that way.
If you decide to confront, which the more I think about, is what I recommend, she is probably going to have a fit and accuse you of all kinds of things. Just look her in the eye and realize that you're dealing with an angry child. Try to remain detached from all of the drama. Enjoy the show.