Look for rust and rot on the body and frame (lift the carpet if it has it). Look to make sure there are no cracks on the frame at the steering box. The standard stuff: fluids (clean no bad smell or signs of water), 4wheel drive hi/lo both work w/o noise or binding, brakes/unit bearings good, leaks of any kind (specifically pinion and rear main engine seal), clutch functionality (if so equipped), all lights work, wiring isn't all hacked up, all features function correctly (wipers, hi/lo beams, dash lights, fans, turn signals)...etc, etc!
Don't let him drive the price on you for those pocket flares and nerf bar. Check tread depth so you know when you will be looking at the 1K for new tires. Glass is all good. Will it pass inspection if your state requires it (smog/safety).