1. Have a friend or the owner help you raise the ATV up and stand it on the rear grab bar. This will allow you to inspect the undercarriage for any serious dents or damage.
2. Look closely at the frame, especially the shock mounts, A-arm mounts and intersections of frame components for any signs of rust. This is normally a sign of a stress crack and would require welding and repainting.
3. On 4x4 ATVs, inspect the CV joint boots. They should be in excellent condition. Any crack or tears will allow water and dirt to enter and cause costly repairs.
4. Also on 4x4 ATVs, if possible remove the front and rear differential inspection plugs. If the gear lube looks like chocolate milk the oil is contaminated with water and there could be damage to the bearings.
5. Check the engine oil to see whether it looks contaminated.
6. Shine a flashlight into the gas tank and look for rust. Yes, a gas tank can rush, and rust can cause carb problems.
7. Raise the ATV and support the front of it with jack stands. Then try to move each front wheel in and out from top to bottom and side to side to check for worn wheel bearings and ball joints. Do the same for the rear.
8. With the ATV on the ground, move the handlebars back and forth. This will identify worn tie rod ends.
9. Remove the seat and take the air box lid off. This is often overlooked. An air box can collect water and damage an engine. A clean air box and filter is a good sign.
10. Check all head lights, tail lights, the engine shut off switch and key switch.
11. Check the hand, foot and parking brakes. Do they operate smoothly or stick? Check for damaged or cracked cables.
12. Check the exhaust. Has the spark arrestor been removed? Almost every state and national forest requires that you have one. You don't want to get to a forest and then find out you can't ride because you don't have a spark arrestor.
13. Check for worn out tires or tires full of plugs. It's not really a problem, but worn tires or plugs should be a negotiating point on price. And the tires should be replaced with new ones. If you have never taken a tire off a wheel, don't do it. They are extremely hard to remove. It's much easier to remove a car tire from a wheel than an ATV tire.
14. Look closely at the front or rear sprockets. Each point should be uniform. If they are worn more on one side or have a hooked appearance, a new chain and sprocket set is in order.
15. After you buy a used machine, it's a good idea to change all the fluids so that you know you have fresh oil, etc.
16. Ride the machine.

Thanks for the help everyone, but it turns out he doesn't have a title or a registration for the Wolverine.