To test a tap .. good advice above. I have also placed a bit of moist tissue over the closed tap on a loaded gun and fired it. A leaky tap will blow the tissue off. Or you can dribble a drop of oil around the tap sides .. a leaky tap shoots the oil away. I have heard about people using talc powder for this but was warned away from talc because it can be an abrasive if not all cleaned away.
Cheers, Phil