Ask the seller to remove the side panels so you can see the "guts".
Turn them on high. Ensure every jet is actually firing with strong pressure. Weak flow could mean air blockages or brittle, failing internal plumbing. The Deep Dive: Under the Panels what to look for when buying a used hot tub
Look for "gentle weeping" from seals, mineral deposits (white crusty spots), or damp insulation. Leaks in fully insulated tubs are notoriously difficult and expensive to fix. Ask the seller to remove the side panels
Look at the base for signs of rot, water damage, or "unwanted guests"—holes suggest that rodents might have nested inside during the winter. mineral deposits (white crusty spots)
. If it struggles to climb, the heater may be corroded or near the end of its life.