The only sure way to know is having sex with him, other than asking him some questions about his experience sexually with a girl. Having sex with him, if doesn't know what he's doing, that's a pretty good sign. If you ask him detailed thing about intercourse, and he can, or refuse to answer, that's a pretty good sign. Guyes, young guys like to make girls think that they are experience and have had sex before. Quite often it's just talk.