[2000mc]

Without actually smoke testing it, I don't think you'll turn up more than a good visual inspection would. If you don't see anything, the vent valve is always a good suspect, you could remove it, jumper power to it and see if you can blow through it. Same could be done with the purge valve, but they're less common, and typically set a code for "flow during non purge" or something like that, but I've seen them set a small leak code too
Another easy check, look at the seal on the gas cap, I've seen some original caps on vehicles newer than yours where there were big chunks gone from the seal due to cracking