It's pretty much impossible to see those back molars without special equipment (and pigs don't enjoy it that much either!) You would do better to check for other signs of overgrown teeth, like drooling, not eating, or making faces when they chew (they will pull their lips back in an exaggerated way when chewing- not just occasionally if something gets stuck, but regularly.) Really, the best thing you can do in terms of checking their teeth is to monitor their weight, as pigs with dental issues will lose weight.
What I do with my pigs (one of whom has had dental issues and one of whom hasn't) is to peek at the front teeth once a week or so (this is pretty easy to do, as you can see the front teeth if you look at your pig's mouth at eye level or slightly below.) I also run my hangs gently along the ridge of their lower jawbones to feel for any lumps, bumps, or tenderness (as abscesses often present as a swelling along the jawline.) Hope this helps!