I have looked this up. I spoke with a fish biologist and he said the only way is to cut them open and check a certain organ. I am just curious when others talk about catching splake, maybe they know something the biologist don’t.
You are correct sir....
The only sure way to distinguish a Splake from a Brook Trout or Lake Trout is to open up the fish and count the pyloric caeca, finger-like projections in the stomach cavity.
It can be positively identified by the number of pyloric caeca, the worm-like appendages on the intestinal tract right after the stomach. The brook trout, which is the smaller parent, has only 23-55 (usually less than 50) pyloric caeca, while the intermediate-sized hybrid has 65-85, and the lake trout, the larger parent, has 93-208 caeca (most often 120-180).