If it's inside a cover, you might be able to see the arc tracks. Sort of like a welder leaves, but vefy faint. You'll be hard pressed to find an arc track within a connector though.
What "exactly" have you tried already?
Are you 100% certain that the mechanic knew what he was doing with the cams? It doesn't sound like he did, IF, you did in-fact tell him that the engine was running fine before your cosmetic upgrades. That sounds more like a mechanic that's grasping at straws for an answer because he doesn't have the troubleshooting skills necessary to do his job.
I would suggest un-doing what "you" did, and then take it to another shop if it doesn't fix it.
Good Luck