Blowing into the muffler pressure tube should push fuel out from the feed tube to the carb.

So, disconnect the tube from the carb, and blow into the pressure tube from the muffler end. Fuel (or air, if the tank does not have fuel in it) should flow out freely from the carb feed line.

If you suspect an air leak at the carb, remove it and check the o'ring (as already suggested). Even if you can't detect any defects, cracks, or splits, just replace it. They are inexpensive enough. Then put the thing back together and make sure the carb tube is pushed all the way down so as to seat against the O'ring, apply some downward pressure while tightening the securing screw(s).

CGr.