The standard answer seems to be that only performing genetic testing will guarantee whether a specific Largemouth Bass is Florida strain or Northern strain. "Hybridization" between Floridas and Northerns also complicate distinguishing between the two strains.

Some time ago I asked a question on the www.pondboss.com website and received an interesting answer. The guys at Pondboss (Bob Lusk and Mark Mc Donald) are most knowledgeable and trustworthy. These guys have years of experience with Bass and breed both the Florida and Northern strains at their Texoma hatchery. Here is my question and Bob Lusk's response:
 
"Question:
I noticed that all the Largemouth Bass fingerlings I obtained some time ago have what seems like a thinnish, but very noticeable black line or "trim" on the outer edge of their tail fins. This is and has been visible on all the fingerlings which range from about 2" - 6" in length. These Bass also have a very dark dorsal area. The dark backs (almost black) and the distinct tailfin "trim" are very visible when viewing the fingerlings in the water. These fingerlings were said to be Florida strain.

Largemouth Bass fingerlings (no convincing claims made about these) acquired from another hatchery on an earlier occasion had a lighter dorsal area and definitely no black tailfin "trim". The tail fins were almost transparent in fingerlings of 2" - 4" in length.

In relation to other bass fingerlings of different sizes (from 2" - 6") and some subadult fish of 10" - 12" I noticed and carefully watched in the relatively clear waters of a quarry in our area (Mafikeng, South Africa) it was apparent that most fingerlings did not have the black tailfin "trim". Bass from various sources have found their way into this quarry ...

Any comments on the black tailfin "trim"? Is it just a natural colour variation or can more significance be attached to it (possibly Florida strain genes?)

Answer:
That thin line is absolutely characteristic of Florida bass. Biologists are careful not to come to the conclusion that all bass with thin black lines are Florida strain, but it certainly is a characteristic.

Another characteristic of Florida strain bass is dark mottling dorsally. Looks like camo. Do people wear camo in S. Africa?
Very likely ... you at least have diversity of your bass gene pool. And that's good.".
 
Food for thought and, possibly a quick way to check if those fingerlings have Florida strain genes!

This should also work on mature Bass.

Goedertrouw Dam is probably the only public dam in Southern Africa which has only pure Florida strain Bass with no Northerns or "hybrids" (F1s) whatsoever! Photos I have seen of Bass from this water clearly display the very dark dorsal area and the black-edged tail fin "trim" on the outer edge of the tail. To see a picture of a Florida strain Bass from Goedertrouw Dam, go to www.bigbass.0catch.com, click on the "Photos" link, then click on the "Photos of Bass from Southern Africa" link and scroll down to the 3rd photo on that page.

Best regards and Tight Lines from Soutrh Africa!