Just replace the lower element and if that doesnt help, replace both thermostats. If youre getting any hot water, then the top element is good. If it were bad, it would never heat up enough to switch on the lower element which fires last, hence cold water from your faucets only. In my area, elements are about $9 each, and thermostats are about the same. If you replace the lower element and both t-stats, youre only spending about $36, so its really not worth the trouble of testing. Just make sure yoy flush the tank out well, change the parts and forget it. You'll need an inch and a half socket (if it's a standard unit) to unscrew the element.