Google search: this would have come up with his arrest. This was extremely relevant information he hid and only thought to look for something after he accidentally ratted on himself. The Google result was buried because of how his name was formatted. (with or without middle name, etc.)
STI test: Not a good sign if someone wants to try to get sexual and STD status never comes up. This goes double if he comes up with a reason for no condom. It’s not worth it, there is nothing in it for you. Many tragic outcomes, and it only seems rare because people don’t share this trauma with anyone but their doctor and their besties.
Have a friend or (best) a mutual friend vet his social media: He hid his wedding photo album from her, but everyone else could see it!
Meet his friends: How long would you like to to date someone before you find out he is best friends with his ex, or that he has unrequited love for his female best friend? How long would you like him to put it off because you are the other woman?
He meets your friends: Some guys might seem nice 1 on 1, but lack normal social skills when it’s anything scenario besides desperately trying to obtain sex. This might only become obvious if you see him interact in another social situation besides conversation on a date.