Brad let me take a stab at this one. I've had the opportunity to catch a number of spotted bass in some very popular waters.
There are actually several subspecies of spotted bass and depending on your locale you might have landed all three if your a tournament angler.
Your right about the appearance being similar to the largemouth bass. But immediately after landing one the red flag goes up. "Attention this is not a largemouth."
Besides the apparent mottled belly and the broken blotches near the midline of the body. The spotted bass has scales on the base portion of the second dorsal fin and its first and second dorsal fin are visibly connected. I've also noticed the upper jaw does not extend past the eye. This has been widely reported as one of the better ways to tell them apart from LMB. Observation above the lateral line shows a number of dark markings, and below the lateral line the scales have dark bases blended with small spots.
