what is the easiest way to tell the difference between a largemouth bass and a spotted bass?

Spotted have smaller mouths. Also color, but thats harder to do without comparison.

Largemouth Bass

Cheek scales are the same size as rest of body scales

Green with a dark horizontal stripe

No rows of dark spots below the lateral line

Upper jaw extends beyond the back of the eye

Smooth tongue

Spotted Bass

Cheek scales are much smaller than the rest of body scales

Green with a dark horizontal stripe

Rows of dark spots below the lateral line

Upper jaw does not extend much beyond back of eye

Tongue has rough patch most of the time

Also in addition to the above, if you look on the bottom of a spots jaw it will have spots in a row and a Largemouth wont.

See http://www.outdooralabama.com/fishing/freshwater/fish/bassblack/spotted/

The best method to determine if the bass is a spotted bass and not a largemouth bass is to feel the tongue. Spotted bass have teeth on their tongue, largemouth don't. The connected dorsal fin can be difficult to confirm on some spots and coloration changes with water chemistry. If you are fishing in Texas, the Guadalupe bass also has teeth on it's tongue, looks like a spot, but this bass isn't a common fish.

Basically if it fights like a smallmouth and looks like a largemouth; it more than likely a spotted bass.

WRB