Electronics project site Hackhut user Badwolf recently bought several cheap mechanical watches from eBay. Inexpensive mechanical watches are known to gain or lose some time each day, and he wanted an easy way to see how accurate his watches were.
His Pentax GPS camera had an intervalometer that can take photos every minute for long periods of time. To check the accuracy of the watches he merely set the camera up in front of all three and took a photo per minute for 24 hours.
This begs the question of how he knew the time on his digital camera was accurate. To verify this he went to the US Atomic clock site and took one-minute interval photographs of the site's clock until he was satisfied that his camera was indeed accurate.
The results of his experiment were that in one day watch #1 lost 24 seconds, watch #2 gained 2 minutes, 48 seconds, and watch #3 lost 24 seconds. He returned watch #2 as not being accurate enough.