Heather Corinna replies:
I'm with my boyfriend for over a year and he's always asking me if I'm a virgin and of course I am. But how can I tell that he's always a virgin too?
The exact same way he can tell about you: by asking him. Then you take one another at your word. If you don't feel you can, that's another issue entirely.
I'm not sure why he needs to have asked you any more than once, unless the two of you aren't dating exclusively, and are seeing other people. If that's not the case, I think it's time to graciously ask that he stops asking you: after all, you're not going to go to bed not having had a certain kind of sex one day, and wake up in the morning, alone, with that having changed unless you went out that night and HAD that kind of sex.
And if you're only seeing each other, than the only person you'd be having it with is him, so he'd hardly need to ask you about it. After a year together, there should be a good lot of trust between you, so asking you about something repeatedly like this isn't speaking to that trust.
Understand, too, that virginity isn't a medical or physical condition: it's an idea, and one which not everyone has the same ideas about. But no one can tell if anyone is a virgin or not just by looking at them or their genitals or by having some sort of physical examination, male or female.
I'd also suggest you ask him why he feels he needs to keep asking you about this: maybe you two DO need to work on the trust between you, or maybe you need to talk about why he feels like knowing the once isn't enough.
Mind, trust doesn't protect you from sexually transmitted infections: with a couple STIs (like oral herpes or public lice), you can pick them up without having had any kind of sexual activity with anyone at all. So, if all this talk about virginity is because you're starting to become sexually active in some way, y'all STILL need to get STI screenings and use safer sex for genital sex to protect yourselves, okay?