How to know if he is really over his ex?

The guy I am talking to broke up with her about 4 years ago but they are still friends. She was his first everything and I feel like he is truly not over her. I had asked him if he is over her , he told me that he got over her after he got into a rebound relationship but i don't really believe it. I haven't dated anyone before so I don't know if I am another rebound of his. What signs should I see if he is really over her or if he deep down still likes her?

How to know if he is really over his ex?
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