There are a couple of key points in making certain that your 1921 and 1921-D mercury dimes are authentic.  In addition to make sure that the D has not been added to a 1921 to make it a 1921-D, look for the following on your 1921's:

Genuine:

• Genuine coins will have t hick 1's with concave sides and very slight serifs. 
• The 9 will have an OPEN loop.

Falsely Altered Dates:

• These will have a straight 1's with remarkably pronounced serifs,
• The loop on the 9 will be closed.

Fake D's

• Fake D's have been added to some 1921's.  The genuine 1921-D will have tails on the serifs extending to the left of the D, whereas the fake D's will have no serifs.

The altered dates will appear in virtually all levels of condition, particularly from Fine to higher grades.