How does one differentiate between a PROOF coin and a well struck Mint State (67-70) specimen? That is if the owner of the coin is unsure of it's origin or is of dubious character? Used to be Proof Coins stayed in the Proof Set so they could be guaranteed to be Proof. Bone

Proof coins have a mirror finish that mint-state coins do not. If you look at a proof coin next to a business strike coin, you can see the difference. Below is a picture of a business strike SBA dollar next to a proof one I found in circulation. Charlie

Some times when the proof doesn't have the CAMEO or a real good mirror to it you can look at the reeding on the rim....or the strike.... Speedy

As has been said, the easiest way to tell a Proof coin is by the fields being polished. But you can also look at the devices and legends of the coin. Since Proof coins are always struck at least twice, the coins typically have much more well defined legends and a much stronger strike. The edges of the letters & numerals will be sharply defined - almost squared off if you will. While the edges of the letters & numerals on a business strike coin will appear to be rounded off.

Usually, the objects are "frosted" in appearance, while the field is mirror-like, in the proofs I own, or have seen. There may be exceptions I am unaware of.

Sometimes proofs are very hard to identify. Many modern proofs bear a mintmark different from that of a business strike. Many earlier coins have the same mintmark as business strikes. The US and world Mints have experimented with many different kinds of finishes over the years. The US Mint has had satin proofs, roman finish proofs, etc. Satin and roman finished coins are very rare and were only made for a few years during experiental die trials. Cameo proofs in the mid 20th century are rare and have a premium over normal proofs. Proofs usually show much more detail than business struck coins because they are stuck many times which allows the metal to flow into the tiny crevasses inside the die.