Just like any used vehicle purchase, check the fluids. Power steering fluid should not smell burnt, brake fluid should be clear, engine oil should be at the correct level on the dipstick with no obvious contamination, and automatic transmission fluid should be red, at the correct level, and not smell burnt. The coolant should be green with no oily residue, and no fuel smell (either can indicate a head gasket problem). Unless the seller was obsessive-compulsive about maintenance, at least one of these checks will fail. Don’t sweat if one or two checks don’t pass. But if the Jeep fails all the checks, you might want to think about moving along.