Well without a evap tester or 5 gas analyzer it's alittle tough, but can be done. I'm a smog tech in California, and have diagnosed most of them without having to resort to a pressure/smoke tester.
First, visually inspect all the hoses from the fill to the canister. Luckily, GM has hard line down the frame rail, and up to the canister. Unless that has rusty holes in it , I've never seen them leak. Leaving only about 3 or 4 inches of hose at the canister. Pull the bed, and disconnect the fill neck from the fill well, leaving the neck on the hose. Check the fill hose for cracks, and the tank vent hose from the pump module to the hard line on the frame.
If you find nothing there, now it's time to use your nose. I'd just use the gas analyzer and look for high HC readings waving and poking the probe around. But you probably don't have that optiion. Start smelling around the top of the pump module for fuel. It helps if you do it on a hot day.
If none of that bears fruit. Take off the lock ring on the pump module, pull it and inspect the O ring.
If cracked/split replace.
Most of the time, on GMT 400s, I find the module seal, O ring, is cracked or torn. The #1 reason that happens, besides age and mileage. Is that someone replaced the pump module, and did not lube the O ring during installation. Or forgot to clean the O ring groove on the tank, and the base of the flange.
The O ring ALWAYS needs to be lubed with SILICONE grease, prior to installation. Do not use white, or wheel bearing grease, silicone only. Not spray, grease.
If after inspecting everything, if you didn't find anything, the it will need to smoke tested. Also, don't just replace a tank out of hand. I've never seen a tank leak unless rust or collision damage put a hole there. Also, most tank leaks would show during filling or leave a puddle.
Anyone in California too, the state has a Repair Assistance Program. Too help people fix their car if it fails a smog check during registration renewal. They'll pay up to $500 in repairs. You can check it out
@ BAR.ca.gov under the Consumer Assistance Program.