It will be extra shiny and the rim will look sharp and squared off. Some will also have a Cameo look to them. Google images of a proof coin and then a business strike of the same coin and year to compare. John1
The last S mint cent in circulation was 1974, so a 1975-2015 S mint is 100% a proof.
Strike quality and cameo or field reflectivity are telling, but a *very* heavily circulated proof can be more or less indistinguishable from a business strike. Proof coins rarely circulate, and even fewer circulate to the point that they no longer look like a proof.
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