Most of us know about the classic way to check for a gas leak on your keg. Fill the keg, pressurize, then spray some star sans liquid on the lid and ball locks, looking for bubbles. While this will often work just fine, it turns out it's very difficult to find a leak on the gas post while the QD is snapped down on the gas post ball lock.
I read a post today from Scott Janish mentioning the use of a spunding valve as a reliable way to check your kegs for leaks. Here's the method:
Fill the keg with some liquid (water, beer, etc)
Add CO2 pressure to desired pressure level
Install a spunding valve on the gas post and set it to a known value (say 10PSI for example) that you record
Wait 24 hours and check the pressure level again to see if it has decreased
Seems really simple but effective. He suggests testing your kegs regularly to avoid wasting gas. Here's more info from homebrewfinds.com