You can just take the diffuser off of the end of the tubing and set it in the water. If you see bubbles going straight to the surface... you're good to go
if not... There's a leak!
If you have a leak, try miniature bulk heads like this:
Just drill a small hole in the top of your 2L bottle and screw these in very tightly. It'll make switching your mix easier and also will help prevent leaks. You can also put some silicone on the ends of them as well before you screw them in to give you an even better seal.