If they are really impaired, you won't see the mirrored fields (I have some like this that I have found). So, the S mintmark is the best way. There is some differences in the lettering as well, but the mint mark will always be an identifier for halves.
This is really a good question. I am too lazy at this point to look up and reference a post I put up in March. I got a roll of halves from a bank that were all proofs. In some years (like 1976) there were clad proofs and 40% proofs. I can be difficult to tell. If you weigh them you can tell. That is how I determined which were which.