How can I tell if I have a clogged fuel evaporator line?

I own a shop, and think this; The best way to tell is to either blow through the line, or use compressed air to blow through it with. If its clogged it will indeed cause pressure to build up inside the gas tank, thus it makes a whooshing noise when you remove the gas cap, but a charcoal canister that is saturated with fuel will do this too. When filling with gas; If the pump clicks off, then don't keep adding fuel to round out the numbers on the pump, as this will only push fuel up into this line, and saturate the charcoal filter. If its saturated, then the only thing you can do is replace it.

Glad to help out, Good Luck!!!