How To Test for Air Leak? (found - tree root grew into line!)

[mickey4paws]

Is there any procedure for testing the sand filter and pump for air leaks? Would using soapy water be the way? Any help would be greatly appreciated. The pool is not losing water. We are looking specifically for an air leak. Our pool has suddenly become really foamy (a blanket of foam is across the whole pool when the filter is running). All chemicals are in range. Thanks.

One other question: Is it okay to run the sand filter with only one valve open (we would have the pool drain valve open, and the leaf skimmer valve closed)?