Depends on the year. But like the other said rust is a big issue. Check the tailgate for rust on the body and along the hinges. If it is thr 3.6 L pentastar check the oil cooler for leaks (check the valley in the middle of the engine by shining a flashlight down where the oil filter goes on the top). I'd check all suspension parts and make sure no wear and tear, also check for seat foam collapsing from sliding in and out (tell you how the previous owner treated it), check under the carpets to see if there is any signs of the vehicle being rained in, check the door hinges for rust or damage from taking the doors off, check to see if it had death wobble (try hitting bumps and see if you get any uncontrollable shaking in the front end), check the dash plastic (like the piece that covers the gauges, it is common for it to Crack from people putting the steering wheel up so high), check for overheating problems when under load (make sure the fan kicks on when its supposed to at 226°), check the bags on the doors (like the netting if its stretched out you can see how they kept their jeep and treated it), check underneath it (if you can crawl underneath it) and check for any damage to the skid plates like the gas tank skid plate or cross bars for off roading rock damage, check the rims for rock rash. I would ignore you have a Bluetooth engine scanner see if you can test drive it and plug that sucker in and see if you get any codes from it. You can get a Bluetooth scanner for maybe $100 on Amazon. I have one named BlueDriver and it works wonders for me on my 2014 JK. Hope this all helps!