How do you know if you have a 1975S penny proof?

To determine if you have a 1975-S penny proof, first, check for the "S" mint mark located on the obverse side, indicating it was minted in San Francisco. Proof coins are characterized by their high-quality finish, featuring a mirrored background and frosted details. Inspect the coin for its sharp details and lack of blemishes, which are typical of proof coins. Additionally, you can compare it to known examples or consult a coin grading guide for further verification.