It might freak her out, which might cause a defensive reaction (I hate it when the dentist pokes and prods at my mouth, too!)... but after it's over and done with and she's had time to de-stress, I don't imagine that she would be any more aggressive than normal.
You kind of have to hold the snake's head between your thumb and pointer just behind where the jaw hinges, so kind of on the side of where those two visible lumps of the skull are, if that makes sense. . Then use a small, sturdy sterile object to open the mouth.
I did this with my male when he stopped eating to make sure everything was okay. I used a Q-tip (not the ends, the long part). Since then, he's been very head shy. Don't do it if you don't need to. I feel like I had a good reason to and I still regret scaring him like that.
I use this technique with a q-tip i that i cut the cotton parts off i feel the cotton always end up getting stuck in the snakes mouth.. Have to be carefull of that if you do use cotton q-tips you can cause allot of damage.