"You" probably don't, lol. Take pics if you try... (jkg) It's not easy to check their teeth- easier to watch for symptoms, & also look fast when they yawn. To deliberately open their mouth with a tongue depressor (sideways) or something similar is awkward & could do more harm than help anyway. Sometimes a tooth will come loose & stick out the side of their mouth or cause other problems, but that should be fairly obvious.
One other thing to remember- snakes see our motion but don't accurately recognize us just visually. They NEED our scent, or touch in some cases (depending on the type of snake & individual personality) to identify us. Seeing us approach just begs the question for them: food? or predator? They're not wired to be "pets", & they're independent...they're not thinking "buddy". They rely on instincts to survive- so that means constantly thinking "food or danger"". It's up to us to show them we're neither one. They're interested in being well-fed, and in feeling safe. We can do both, & should.