(note - this posted at the same time as Thomas!)
Hi stjohn, thanks for your kind words!
It's a great idea to do what you're doing, although you may be restricted deopending on where the shop will let you test the lens.
In terms of aperture, you can find out a lot about a lens when it's wide open (smallest f number) and when it's at its optimum aperture which is usually f8.
Try to take the same photo using a tripod at the widest aperture and at f8 of a subject which has fine detail in the corners (but make sure it's in focus too as when wide open you'll have a small depth of field).
Then presuming you're using a zoom, repeat at different focal lengths, say at the widest and longest and something in the middle.
You could also take a photo of a plain white wall or blue sky to reveal vignetting (dareniong in the corners), along with a grid pattern (maybe a brick wall or even a computer spreadsheet on an LCD monitor) to test for geometric distortion.
To find out how we test lenses visit this page:
Forum Moderator Thomas has also done many lens tests in the forum and describes his methods in posts like these: