Keel bolts hold several tonnes of ballast to the hull, and almost all of each bolt is out of sight: through the keel stub, inside the joint and in the ballast. A surveyor sees the nuts in the bilge and the joint from outside. Most keel bolt findings come down to reading those two places well and knowing when to say "pull a bolt".
How a surveyor checks them
With the boat out of the water, the joint between keel and hull comes first. Practical Sailor's signs to look for: large gaps in the joint, rust weeping near it, and crumbling bolt ends in the bilge. An open crack at the forward or aft end, the smile, may be filler or may be the keel working.
Inside, the surveyor looks at each nut and backing plate, taps them with a hammer, and checks the bilge around them for cracks in the floors and stringers. Practical Sailor is blunt that looking at the nuts isn't an inspection, because corrosion works where seawater sits, inside the joint. One clear sign: a nut that has dropped lower at the top than the bottom means the bolt is wasting away under it.
World Sailing's model keel inspection, in Appendix L of the Offshore Special Regulations, adds checking the nuts for corrosion and the bolt holes for ovaling. The builder's torque figures are the reference for tightness.
When to pull a bolt
When the signs point to the bolts, the only way to know is to look at one. Practical Sailor notes X-ray can work for steel and stainless bolts and is almost useless for bronze. A pre-purchase survey rarely includes pulling a bolt, so the finding should say what was seen and recommend it.