Possible Return Line Leak

I have an inground qunite pool that just began loosing about an inch of water per day. I've done the bucket test comparisons - only loosing water when the equip is running and I'm running my equip about 6 hours a day during this winter. Outside of getting a leak detection company to come out, what is the best way to determine where is leak is occuring? I have a rather large concrete deck that is slightly sloped and I do see a small amount of standing water building up a the low end so I am thinking it has to be one of the return lines. Which one and exactly where is the question. Thanks in advance for your help.