You can't tell if you're sexually active just from looking. Given active lifestyles these days, it's not uncommon for a girl's hymen to break long before she's ever sexually active.
I don't know about obvious signs that you're sexually active (apart from seeing symptoms of a STD, pregnancy, or sperm), but can't you just request a different gyn? It's awkward enough, but I think it would be more so to have my mom do it.