Basically, start with the visual items - cleanliness of the unit, water stains on inside walls and ceiling, etc. Push and prod with your hand on the inside walls for soft spots. A soft spot could indicate a water leak and a damaged or rotten interior wall. Then check and see if all equipment functions - refridge, a/c, stovetop, oven (if equipped), etc. Does the unit raise and lower properly, visually check the lift system for hydraulic leaks (puddle under the trailer), etc. Check the lift cables to see if they are in good shape and not frayed. Lastly, check the seal between the upper and lower body halves for tears or damage. These are just general things to check out and not a specific guideline. Good luck.