How do I know he's sincere?

Hello.
How do you know for certain that someone is sincere? How do you know they mean it when they say they miss you?

Of course, I’m asking this because there is a guy. Late last year, some questionable stuff that I couldn’t prove happened (involving girls), and so I decided to cut him off early because of uncertainty. I had no proof, and he denied/defended everything of course. Note that he wasn’t officially my boyfriend and we weren’t sleeping together…we were just talking, having some romance, and he wanted me.

I resolved to start the new year without him, so I haven’t contacted him since 1st January. However, yesterday sent me an email, and last month around Valentine’s, he sent me a few emails, which I was strong enough to ignore. Note that I had blocked him on socials, he just found me through email.

He says he misses me and would be happy to hear from me. Keeping no contact is becoming too hard as I think of what could have been and could be. It’s taking me all my strength to not reply back, and at this point I’m losing my motivation for ghosting him.

But will I be delusional if I give him just one chance? Just one? How do I know the “I miss you” messages are real? What if we would have had a great relationship if I had given him a chance? Or should I break off the no-contact and tell him we just be friends? I feel so blinded right now and don’t know what to trust, which is why I’m turning to you for help.