You have a wonderful begining on your test. Put the gauge on the line, air it up to 15# and recheck the gauge 24 hours later. If it has dropped 2 or 3 pounds you probably have a leak. Mix 1 part water with 1 part dish liquid (I would use Dawn because of its grease cutting power), spray all the joints and fittings (iron pipe fittings are cast iron and may have a pinhole in them), and watch for bubbles or foam (it could take 15 minutes on a slow leak). Be careful about putting too much air in the line because compressed air in a pipe has stored energy; it doesn't just pop off, it comes out at a high rate over what would seem to be an extended time. Also 1/2 to 1 pond drop in the line may be becuase of tempreature change.
A plumbing inspector friend of mine says "If it is holding, it'll hold for a week; if it has a real leak it will go down in 15 minutes".