I still maintain there are many nice or "early strike" currency coins being wrongly passed as proofs, with the consequent massive uplift in cost for potential customers.
I'd imagine these are a mixture of genuine error and deliberate cynical attempts to deceive. Virtually impossible to distinguish between the two, hence the fraudsters get away with it. Especially if they've also fooled NGC.
My own rule of thumb is that some coins hit you in the eye as very obvious genuine proofs, as soon as you see them. I'd stick to them. Alternatively, some you know that cannot be anything other than a proof, such as the 1839 mentioned earlier, the KP31 1806 copper, bronzed copper and gilt proofs with the incomplete 0 & 1 in the date, and the R97 & R98 Taylor re-strikes with the tiny collection of rust spots at the base of the second A on the reverse.
Slightest doubt, steer clear.