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.
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.