The biggest area of complaint for any lens seems to be front focus or rear focusing.

Which means the camera is not focusing on the exact spot you have chosen as the target.

You can get special charts for testing this, but here is what I did.

It is best to use room lights and no flash.

Place a yardstick on the floor with the lowest numbered end closest to the camera.
Adjust you camera to use center focus.
Place the camera on a tripod looking down on the yardstick at about a 45 degree angle. The camera lens should be about 5 feet from the center of the yardstick.

Focus the camera at the 18 inch mark on the yardstick.
Using the automatic focus.

Take the photo and view it on your computer.

The 18 inch mark should be sharp and clear.
Due to depth of should see the numbers furtherest from the 18 inch mark to be blurred somewhat.

If you find (lets say) that the 9 inch mark is the sharpest, the lens is front focusing.

You can change the effect of the depth of field by increasing/decreasing the light in the room or by moving the lens closer/further from the target.