I second Leah's tip with the cotton swabs. My snakes are on paper towel so I don't have a problem with substrate ingestion, but I occasionally use that method to check for illnesses like RI.
I place my thumb behind the head, kind of where the jaw hinges if you know what I mean, and then my forefinger in the same place on the other side and gently push the bar part of the Qtip horizontally in between the jaws from the front of the mouth, they open right up! (Sorry I'm terrible at explaining things like that, hope you got it)