Start with the discipline: pool, snooker, pyramid and carom cues are built around different playing needs. Then check length, weight, balance, shaft, joint and tip. Hold the cue in your own stance; it should not make you grip harder or compensate with your shoulder. A joint should close smoothly without play, and the shaft should be free from cracks and rough spots. For a used cue, request photos of the joint, tip, ferrule and serial marking. Links: Cues, cue tips, used-cue checklist. Sources: WPA Equipment Specifications and WCBS Glossary.
Before buying, match these checks to your playing goals and ask the seller to confirm the exact model specifications.