It depends on what you have as not all apply in all instances. Generally speaking therefore, a proof will have brilliant fields, square rims, sharper milling, squarer sides to the lettering, a better quality of engraving to the detail and is likely to be in close to as struck condition. Although a few proofs get circulated, the vast majority do not, and in the case of non-set years, almost never. Easiest way is comparing a known currency piece with a claimed proof and then look for reasons why it isn't one rather than saying it is better, so must be one.