[CT]

A quick check I find is to mount the camera on a tripod and set it up facing a brick wall outside where there's plenty of light. There's lots of fine texture in the bricks and mortar, and it's all on one plane. Make sure the camera is square to the wall. Take a series of shots at different zoom settings and apertures. f8 tends to be as sharp as it gets as after that you get diffraction effects fom the light bouncing off the diaghragm blades.

Make sure you release the shutter with the delayed timer - preferably a cable release. Use mirror lock-up if you have it.

Not conclusive or scientific but should put your mind at rest ... or not!