As said above, it's OK to have some bubbles if it's hot outside. It's OK to have a lot of bubbles immediately after the compressor turns off, but not while it's running at a steady state. The two ways to find leaks that I'm aware of are to 1) get a sniffer, or 2) add an ultraviolet die to the refrigerant, let it run for a while (come back in a few day's if it's a really slow leak) then crawl around under the hood with a black light and look for the glow.