Coins with 'H' or 'KN' are easy, even on worn examples the date has a small 'H' or 'KN' to the upper left. Modified effigy on 1926 is very difficult to spot on worn coins, with the modified effigy the BM initials on the truncation (bottom) of the Kings neck appear just as 'BM' to the right of the truncation. With earlier un modified head pennies the BM appears as 'B.M.' more in the middle of the truncation. 1945 double '9' i've never looked out for, but I would imagine you'd need a good magnifying glass (just like the one I sell!) to spot a double struck '9' which will appear as 2x '9's on close examinination, one on top of the other. 1953 Proofs all look very prooflike, with mirror like fields. They are not that common in circulation.