Best is the gray matter between your ears. Most leaks can be found by looking over the system carefully.
Then there's the hard ones...UV dye works well for most of these.
My favorite dye light I made by accident. I had a 2xAA LED Maglite that suffered intermittent connection, making it blink out. I disassembled it, and sprayed it off with either electronic parts cleaner, or brake cleaner (I forget). When I reassembled it, it no longer had the intermeittent connection, but it only put out a faint bluish purple light. I later realized it was UV. It will make UV dye glow much brighter than the UV dye light I bought for that actual purpose (plus that one had alligator clips for 12v battery. AA cordless is so much more handy).