mine was obvious by the pectoral fin, snubbed and worn out. You can see it in my pic. it's not hiding behind my fingers, that's actually how tiny it was. It's kind of surprising how these guys swim without the big pectoral fins; it might explain why this one was not in fast water.
I also found by color which ones can be stocked. Im not 100% on this, but the stocked ones I catch are have more red and brown overall.
The wild ones tend to be overall lighter, red only where the dots are, a smooth tan to silver to yellowish bellow. In addition, stocked browns can have big, messy, black dots.
Wild one:
i think fins grow back, so some stocked holdovers will grow out their fins but still maintain those same colors.
But other than the fins, Im never 100% on stocked vs wild.