There's a couple of ways to tell the difference. I'm far from a bass expert, so others should chime in. But one that I know of is the ratio of the mouth to the fish, which sounds obvious, but is in fact a means of determining the species. On a largemouth, the hinge or corner of the jaw extends to even with, or just aft of the eye when the mouth is closed. For a smallie, the hinge is forward of the eye when the mouth is closed.
Which you can see here-
Likewise, the largemouth with their typical black line down the side, and the further back hinge-
Good job Sox. The mouth hinge is a great way to tell.
90% of Smallmouth will have a piercing red eye. A largemouth's will be a golden color.
The BLACK horizontal stripe is what really stands out the most though. You'll notice, once you start catching a lot of both, that the Smallmouth are either brown or green, but you won't see a horizontal stripe. The largmouth are typically a greenish/white, with a noticable Black Stripe.