Like he says , a bit of fuel ( or water , meths , etc . Scotch is probly safer , AS . . . ) put a piece of tube ( a few foot ) on it ,
Bowing Gently and tilting it about , should get to the core of the matter , or end of the tubes .
Blowing or Sucking ?
Blowing youll get bubly noises while the pipe your ons submerged . Dont get it in the eye .
Sucking with say 10 ml in it , youd find the pipe ends when theyre under it / submerged .
Do a sketch as you go , the fumes can induce false arithmatic .