[splink48]

The technical way is to take a pressure reading off the system while the car is running and full of coolant. If it reads 16 lbs, the cap is good.

The redneck way is to take off the cap, examine the seal to see how good it looks and make sure its not dryrotted or cracked, then push the metal tab at the bottom to ensure the spring isn't locked up. If you're applying more than 16 lbs of pressure, there's a problem, if its way less than 16 lbs of pressure, theres also a problem.

I doubt the water is leaking out of the cap...you never know, but I sincerely doubt it. It could be evaporating in the passages and seeping out, but you would probably have smoke either out the tailpipe or under the hood while driving. Keep in mind that the water goes somewhere, it doesn't just disappear, so as long as you're losing it, you've got some kind of problem.