There is no size limit on spots. However, they are included in the 5 bass per day per angler creel limit.
The dead giveaway is the tongue patch. A small, rough, usually dark patch on the tongue. If the tongue is smooth and the fish is less than 15" long, THROW IT BACK, QUICK. IT'S A LARGEMOUTH!!!
Let's look at a couple pictures from the Weiss Tx.
The first picture is left to right: LM, spot, spot, spot
The second picture is LM on left spot on right.
Notice how the spots are a little more streamlined than the Largemouth. The spots also have dark splotches across their backs. And the upper lip of a Largemouth extends behind the eye, where a spot's lip comes back even with the eye.