How do I know he is single

All I will say is tread carefully. TBH it's better to just assume every man is commited/married (esp. depending on the age range, esp. if he's 30+), until and unless you have 100%, verifiable proof otherwise.

Just ask - "is there anyone who thinks they are in a relationship with you?"

You can never be 100% sure, but I would say a very surefire sign is if you have his undivided, constant attention. His attention isn't elsewhere with other women (which us women have an intuition for anyway), but only you. It's not something you can pinpoint to exact, specific numbers - you just feel it. He replies immediately or at least very fast, doesn't leave you on read, doesn't ignore you, you feel his enthusiasm for you. You feel like he's 100% into you. I experienced this last year with a guy and I was 100% certain there was no other woman because I had his attention wherever he went. He would always make sure to check in with me, always wanted to hear from me...I could literally feel there was no one else, simply from his behavior. And of course, towards the end his messages became less in frequency, shorter and less enthusiastic.....I suspected right away he was giving his attention to other women then (which he denied and gaslighted me about) and was proven right in the end. So yes, that's my number one sign. You feel it. Trust your intution, always.

Ask on your local FB group, “Are we dating the same guy”