From what I have read, there is no way to tell if a fish is a splake or not by outward appearance.
You need a scientific knowledge of anatomy and a sharp knife.
A few years back I was fishing a splake lake between Christmas and new year day when trout season opens, but splake is year round.
We caught a fish and couldn't be sure if it was splake or spec so we released it
After this I researched a bit and found that it is impossible to determine even for a CO.
Thus,if a lake has been stocked with splake, all trout in that lake are specs and you can't be charged for OOS fish