If her breakup with her ex is relatively recent, she is going to spend time thinking about the relationship. She still has issues to resolve. Why did they breakup? What did she do wrong? That sort of thing. If she walked out because he was unfaithful, he might be trying to fight or grovel his way back. It’s best not to get involved on the rebound.
There’s a reason you think she might be lying. It could be that you are just an untrusting and unreasonably jealous guy. Or it could be because of the symptoms she is exhibiting.
The best indication that her heart does not belong to only you is to notice if her body seems to be all yours.
If she is distant, avoids eye contact, ends make-out sessions more quickly than before, jumps off the couch when you start getting romantic, or is not totally “present” during sex, you might have a problem.
The best way to find out the real truth about your girlfriend and her ex is to ask her.
Her words might be telling you she is finished with him, but her mood or tears or temper are telling you something else. Sit down and have a talk. You will have to be supportive and not make accusations. Ask her if she needs a little rope or what you can do to make things better.
Communication is the key to so many aspects of a relationship. But communication only works when it is positive, supportive and non-confrontational.