A fin keel is a long lever. Hit something hard at speed and the keel tries to rotate backwards, pulling at the front of the joint and pushing up at the back. The damage it does is often inside the boat, under the floorboards, where the owner never looks.
Where the damage goes
Safe Skipper describes it for fin keels: a hard grounding can split the keel to hull joint and force the aft end of the keel upwards, damaging the hull laminate. Splits tend to be outside the hull forward of the keel and inside the hull aft of it.
The keel to hull joint can open into a "smile". Practical Sailor calls it the ever-widening gap between keel and stub; a minor one may need only a light repair, a persistent or corroded one more.
How a surveyor checks it
Outside, look at the joint with the boat on the slings and again on the ground: a gap that opens and closes is moving. Look for cracks fore and aft of the keel, rust staining from the joint, and damage to the keel itself.
Inside, lift every floorboard. Check the keel bolts and backing plates, the floors and the grid or tabbing round them, and the laminate aft of the keel. Practical Sailor's advice is to inspect keel bolts after any grounding. World Sailing's offshore rules require an internal and external inspection after each unintentional grounding, a sensible standard for any boat.
What decides the grade
Structure. Cracked laminate, floors or tabbing, or a joint that moves, is an A. A smile with staining, or keel damage to repair, is a B. A hairline crack in the antifouling at the joint with nothing else is a C.