Yeah, depends on what species. Rainbows are the toughest for me to tell, though that's from lack of experience somewhat.
Browns are the easiest. Perfect translucent fins, blue/black spot behind the eye, red coloration in the adipose fin are all good indicators. None of them 100%, but if you got that trifecta going for it, you are 99% sure. If none of the above, you are 99% sure it's stocked. Then there are tweeners that may have some, but not all. Those are tougher, and quite often you find out they're fingerling stocked or something like that.
As for brookies. The easiest way to tell is size and shape. Stocked brookies are thick and 9-12" long, the product of a fast growth rate. That's extremely rare for a wild brookie, where 4-8" is the norm, and the bigger ones are typically skinny and big headed. But this is really based on growth rate, not origin, so a wild fish in a lake or a limestoner, i.e. richer food environment, may grow quickly and fool ya. Another good indicator is the white and black markings on the caudal and anal fins. In wild fish, the white and black are completely separated and typically by a perfect or almost perfect straight line. In stocked fish, the delineation may be "muddy", or at least the dividing line isn't so perfectly straight.
Of course, location and what's expected to be there comes into it. Of fish that could go either way, the most probable situation for the water in question usually gets the benefit of the doubt.
All physical markers are probabilistic, meaning if you know what to look for, you can be right most of the time, but nobody can say they have never, ever gotten one wrong. There's no 100% marker short of putting scales under a microscope, or doing DNA testing.
