I'm trying to recall the post service checks that Oxyhacker (Vance Harlow) recommends in his book.
1. By hand, try to unscrew things like hoses, covers, 1st stage body. Everything should be more than handtight except maybe the cover on the 2nd which should at least be snug.
2. Hook reg to tank but leave air off. (can also be done by securing dust cap on 1st.) Try to breath thru 2nd stage (not too hard). You should get nothing. If it does fix the leak.
3. Air on & a sink or bucket of water. With mouthpiece pointed up slowly stick second into water. Before the water reaches the top of the mouthpiece and enters the reg, a freeflow should start. If not, something is wrong - might just need tuning. Back off enough to stop the freeflow and make sure no water runs into the main body. No leaks should occur.
4. Air on. Look/listen for leaks. Submerge tank & regs in bathtub. Move it around a bit to release any captured water and then give it a few minutes and look for bubbles that indicate a leak. With the mouthpiece up under water, you don't have to wait for the body to fill with air to find a leak inside the 2nd.
If you do find leaks, try curing by purging vigoriously a couple times. If the problem is a little dirt in the wrong place, this will often remove it. It leak persists, seek technical help.
enjoy