There are two types of 1946, 1946 D, and 1946 S Roosevelt dimes.  

Type I that can be distinguished by the Y in LIBERTY on the obverse is farther from Roosevelt's forelock and (the easier way to tell) the initials JS of John Skinner, the designer, are very weak and misshapen.

Whereas the Type II has the Y closer to Roosevelt's forelock and the initials JS are large, bold, and clear.

The 1946 S and 1946 D Type I  are very scarce.