I don't know how you would ever be able to tell the extent to which someone worked on his car.
And, what level? I don't do paint. But I can sand and fill. I can't sew my upholstery, but I can take out my seats. I can't loom a rug, but I can pull out my carpet.
Maybe every car show entrant should qualify by being certified. You go to a car show certifier, and you are given 10 minutes to change a tire and replace that passenger side spark plug that is a pain in the rear. That get's you to level 1.
If you can replace your disc brakes and repack your bearing that gets you to level 2.
If you can degree a cam, that gets you to level 3.
And if you want a level 4, you have to accomplish all 3 tasks after finishing off a six pack.