Start with the paperwork, not the paint. A combine with a complete service file is worth a premium over an identical machine with a story.
Compare engine hours to separator hours. A ratio far below 0.6 usually means long road runs. Far above 0.85 suggests heavy threshing in abrasive crops.
Under the shields check rotor rasp bar height against factory spec at three points and look for concave wire wear past the witness marks.