From 1964 back, proofs were made at Philidelphia. From 1968 on, proofs are made at San Francisco. No proofs were struck during 1943-1949 or 1965-1967.
You can recognize a proof by its mirrored background. The detail will be sharper and the edges will be taller. Some have a frosted appearance on the details. Look closely at the pic above, it is a frosted proof.