Easy enough from my looking in. Grab a carb making sure that it is install ready, so, no open ports, tape off the bottom (the part that goes into the boot or manifold), make sure it's sealed well, now slowly pour water down the throat and see where it comes out. The throat being the air intake part that is usually at the top. Now when the test is done, you could just turn it upside down to drain or you could punch a weeee little hole in the tape, and time how long that takes. Just a pin hole eh!?! Wanta keep you busy for a bit.