I hold my ball python's head gently but firmly and use a very soft, flexible rubber spatula to open his mouth. I ease the corner of the spatula slowly into his mouth until he opens wide and I can take a quick look-see. He doesn't love it, but the spatula is a wide blocker and allows me a good safe look.
I'm sure others will have additional great ideas like tongue depressors and a variety of other implements... This just works for me.
Best of luck.