This is best identified by noticing the 1942 date punched over a 1941 date. The error from the Philadelphia Mint is far more noticeable than the same one at the Denver Mint. It is also one of the most identifiable and prominent errors in all US coins.
The 1942 42 over 41 Overdate Error (1942/1-P) was one of the major errors in the silver Mercury Dime series. What This Coin Looks Like (Obverse, Reverse, Mint Mark Location, Special Features, etc.):
