theres youtube videos on detecting the leaks on hw100.
as a process of elimination, I would first fill the bottle and remove it, mix up some weak fairy liquid /water.. just enough so it can bubble and paint it all over both ends of the cylinder and see if it bubbles anywhere.
if that is good, I would put a balloon over the barrel and check for leaks through the regulator and transfer port.
if you watch the vids, its pretty easy job