If I had a nickle for every time I have heard this question. But if you are someone asking the question, don't feel bad. I have caught tons of both, and most of the time, I'm just happy to have a bass on the other end of the line. It really doesn't matter. But, like every other aspect of fishing, we all want to hone our skills, and fish identification is part of it.
(Check out the connected dorsal fin in the pic, and view the video to see the tooth patch on his tongue)
There are some distinct differences between the spotted bass and a largemouth. Where the mouth hinges, is probably the one most people use because it's pretty easy to see. If that hinge is past the eye, it's a largemouth. If it's not, then it's a spot (unless of course you caught a smallmouth or some of those fancy pants bass you find down in Georgia, which is a different blog post for the future).
While there are color differences and pattern differences, I don't get too caught up on that, because that is probably the number one way that people misidentify the two types of bass. Just because it has "spots", doesn't mean that it's a spotted bass. It could be a largemouth with distinct markings. You know what I'm talking about. That big fat one you pull out of the lily pads in the middle of August.
With this in mind, in my opinion, you should concentrate on the two easiest ways to tell a spotted bass from a largemouth bass. Dorsal fin and tooth patch.
Dorsal fin is probably the best way to distinguish the two. If you take a look at my video, you can see on my special guest, Mr. Spotted Bass, that the front and rear of the dorsal fin is connected. On a largemouth bass, the front and rear will either not look connected at all, or look like they are barely meeting in comparison to the spotted bass. Lift the fin. If it's connected, you know who hit the bait.
The second indicator, that works a majority of the time, is the tooth patch on the tongue. Once again, it's very distinct in the video. I recently was on a fishing trip with river rat Jason Wolfe, and he commented on the fact that largemouth, will on occasion, also have the tooth patch, although it is pretty rare. Our friends the biologist, seem to back this up, so if you want 100% identification, I'd look at the following.
If it has a connected dorsal fin, mouth that doesn't hinge after the eye, and a tooth patch on the tongue, congratulations, you are holding a spotted bass.