Each Concours is different. But JCNA. Jaguar Concours North America, does have a pretty rigid set of criteria by which the cars are judged and that is really what you want.
You can get a score sheet with a list of exactly what is judged on the car and a list of exactly what the minimum and maximum point deduction is for any particular defect might be. Dirt, rust, dent, chip, scratch, non-authentic, etc.
Then there are rule books to spell out all the "grey areas" so if there are questions you look it up.
These, of course are for concourses that are car-specific and judged by actual elected judges who are normally schooled in the cars they are looking at.
"Popular choice" car shows ar a different matter and it just comes down to showmanship and pleasing the eye of the guy looking at your car. It's a different ball game. The rules, if any, are different at every show.