Background Reading: 3-P: Sealed Bases and 6-H: Rodent infestation.  

Rats and mice can do major damage to hot tub plumbing and wiring, and also can pose a serious health threat to humans. 

Tips:

-In the equipment bay, if the top of the white pipes are much dirtier than the sides and bottom (black/grey), it’s a sign that rats or mice have been running across them a lot with their unhygienic, grimy feet.  
-If the engine bay smells bad, it’s probably a lot worse behind the panels.
-With fully foamed models, check for fingernail-sized chips of foam – these are mouthfuls of foam that rats have bitten out to form their tunnels.
-While you’re at it, check for rat poop, although foam chips are usually easier to spot.
-With fully-foamed models, take the side panels off and check for tunnels and nesting pockets that the rats have burrowed through the foam, sometimes stuffed with sticks and bits of scavenged plastic
-If the hot tub is not fully foamed, check the pipes, wires and hoses for signs of gnawing.
-If the hot tub is empty (more important for hot tubs with non-removable panels), lift the tub up to see underneath and look for dead rats, skeletons and heaps of foam chips.