I agree, a lot of the "leaks" you see are the results of bad workmanship at the initial connection stage, most did not clean up and left the residue as it was.Like KME says, the best way to see if its old or new is to feel the head with the shower, kettle, tumble dryer and anything else you can get to run at the same time, heat due to connection problems will melt the bitumen and you will be able to feel the heat.
If it remains cool during this crude test it will most likely be old residue done at connection rather than leaking overheating head.