You mean something like this? "Clicky" help appreciated!
http://www.walmart.com/ip/Propane-Tank-Gauge/13350316
If you have removable 20# (or 30#) bottles, you can put one of these on your tank. With all propane appliances turned OFF, open the propane tank valve to pressurize the line, then close the propane tank valve. Watch the needle on the gauge. If it drops over a period of time, you have a leak. If it remains in the same place, no leak. Used one like this for many years on our TC. Won't work now with my fixed mount tank tho.. ST