LOTS of chis have underbites. Its really common. Good level bites are hard in this breed because their mouths are so small.
A dog should have 42 teeth. Try fitting 42 teeth into a muzzle an inch long.
That's one reason why dental care is SO important. Many chis have incorrect bites (overshot, undershot, wry) and are missing teeth. The teeth they have may be loosely rooted. So take care of those teeth, no matter what kind of bite they have!
It's easy to tell. If you can see the bottom lip when the mouth is closed, then the dog probably has an underbite.
Oh and yes... Brody has an obvious underbite. As long as an incorrect bite doesn't interfere with eating, it's just an aesthetic thing. You can see his underbite in the teeth photo in his siggy. OH, and he has missing teeth too.
(As do most chi's).