Drive the car. Get it hot enough to cause the hard start problem then let it sit. Once it's had a good heat soak, pull the air cleaner. Do not try to start it. Look down the throat of the carb and pump the throttle. If you see a strong stream of fuel coming out of the accelerator pump then you do not have a problem with the fuel leaving the carb.
If it's a weak stream or won't squirt the same strength 3 or 4 times then you do have a problem with fuel boiling out of the carb.

I don't know how someone can suck fuel from the carb back into the tank. The inlet is at the top of the carb. If there's enough draw to pull fuel across the air in the top of the bowls and into the fuel line, it would have enough suction to cram the needles into the seats. That would cork the hole and prevent any fuel drain back. You also run into the fact that the bowls are vented to the atmosphere and the fuel line "suction" would draw air through the vents and never pull fuel from the bowls.