My ex boyfriend's (which I slept with last week) girlfriend (which I didn't know about until now) just texted me to ask if he's cheating on her. How do I know it's really her and that it's not just him playing a sick twisted joke on mw?

I know you're going to text back and keep the 💩 going. Options

1.  Move on.

2. Text her with 😨.

3. Invite her to Jerry Springer and fight over a man who does want or respect either of you.

4. Have him use a condoms, you really don't want to keep getting her... You know in you. Since neither of you are going to stop as you say sleeping with him.

Respond with the truth. Apologize and tell her that you had slept with him and that, yes, he's cheating. Be sure to mention that you had no idea he had a girlfriend. As far as the guy goes, dump him immediately. Either way, he's a liar and a cheater.

Either way, say yes. It matters not whom it is. Truth remains the same.

The issue is not whether she is entitled to the truth. 

The issue is whether she is entitled to it from you.

Since you didn't know he had a new girlfriend, I would tell her that he is the one she should be asking.

But relationships are habituating and clean cut breaks are not typical.

That may be a mitigating factor, but it's for them to deal with, not you.

Etiher way .. The truth is the truth.  Just say .. Yep, he sure is!

Not sure why it would matter .. He clearly cheated on her, and he hid the truth from you.  He is untrustWORTHY. He is simply not even worth wasting your time with. 

WTF are you babbling about?

This is bad. In the Queens English you should have said "whom" instead of "which" in your parenthesised piece in the first paragraph.