If your unsure the tank is empty, on the shoulder you should see the letters TW with some digits following. The numbers represent the cylinder weight empty, weight the cylinder and subtract the TW.
As for Teflon tape, if it is a brass to brass thread connection it is unnecessary. Two of the reasons brass was chosen for the compressed gas industry is for it's ability to handle high pressure and it's soft metallurgical properties. The soft property make a good fit for mating surfaces like threads.
I chased a leek for quite sometime on my system and come to find a number of my worm gear (hose) claps had small leaks, almost undetectable individually. I replaces all of them with Oetiker clamps and my problem was solved.