I have used the corner of a credit card before to pry mouths open. I really have only had to do this a couple of times, I was force feeding a reluctant snake that hadn't eaten in a long time and I was worried enough about her to do it. It is stressful for them. If your snake is eating ok, most of the time after they eat they will 'yawn' to realign their jaws. You could probably get a good look then. Are you worried about a mouth infection or something?