Check engine light-Venture van

97 Venture van 74000km The check engine light has lit for the second time in two weeks. The first time I had it reset at a local garage, the mechanic said there was an EVAP code set and the system may be loosing pressure somewhere. He suggested the local GM dealer could run a pressure test or a smoke test to see where the system is loosing pressure. Is this likely to find the source of the problem, as the van seems to run just fine as is. I have checked the gas cap !

-- thanks Terry