Elementary, My Dear Wason

This riddle was created by psychologist Peter Wason some time ago as an aptitude test, and only 20% of applicants tend to get this test right. Credits goes to Einstein's Riddle, by Jeremy Stangroom, for the majority of this riddle.

An officer is presented with four cards, and is told that they have been manufactured according to this rule:

"If a card has a circle on one side, then it has the color yellow on the other."

He is then informed that every card does have a shape on one side and a color on the other. To pass the test, the officer simply has to identify which of the four cards it is necessary to turn over, and only that or those cards, in order to determine whether the manufacturing rule has been upheld.

The four cards are as follows:

Which card or cards does he need to turn over in order to check whether the manufacturing rule has been upheld?