Look at the base for signs of rot, water damage, or "unwanted guests"—holes suggest that rodents might have nested inside during the winter.
Lift it. A good cover should be light. If it feels like it weighs 200 pounds, it’s waterlogged and has lost its insulation value; you’ll need to factor in several hundred dollars for a replacement. The "Wet Test": Seeing it in Action
Before shaking hands, ask the seller about the tub's pedigree: What to look for when buying a used hot tub?
Never buy a hot tub that is empty or "dry." You need to see it running to truly vet its health.