Soapy water (bubble blowing liquid from a kid's store) will do just fine. You can also turn-off the LP supply and see how long it takes the flag to pop up. That will give you an overall system test. You can get a gauge that goes in the supply line to test for leaks also. These last two methods will tell you if you have a leak, but not where it is. Use your nose and some soap to pin-point any. It is rumored that some people have checked for leaks with an open flame but their exact technique is unknown since they are no longer available for comment!
"Not all who are laundering are washed" say Bill & Heidi
'78 Excella 500,"The Silver Pullit". vacuum over hydraulic disc brakes, center bath, rear twin. '67 Travelall 1200 B 4X4 WBCCI 3737