Judging the detailing is simply the same aspects you or I would use to view a car. Ask yourself, "What isn't perfect?"
cleanliness overall is paramount.
rubber: consistant and appropriate color, sheen, etc.
chrome: shine, straightness, scratches, etc
paint: straightness, panel fit, consistant finish, paint spray quality, scratches, swirls, etc
leather, cloth, carpets: consistant color, wear, seam fit, shape, texture, etc
glass: cleanliness, clarity, scratches, wear, etc
presentation: arrangement of seatbelts, accessories, wheel position, bolt/screw head positioning, etc
The above is relatively easy to accomplish. The more difficult aspect of Concours is the rules of authenticity. The correct vintage parts, tools, documentation, color, mechanical set up, wires, clamps, hoses, tires, leather, mechanical parts, etc.
I have sheets for judging Jaguars and there is about 200+ points to check regarding all the above. Many cars look great overall but when you get your nose close to each individual panel or area of the car you start checking off defects.