This is the basic checklist I use when looking at a used quad:
The first thing I do before anything else is check to see if the motor is warm. This could possibly indicate that the seller warmed it up before you arrived in order to hide a problem (smoking on start up, cold running problems, etc...)
1. Compression (remember to turn off the fuel and hold the throttle wide open when doing the compression test.)
2. Oil - check for color, metal flakes, etc.. any oil leaks around the head gasket are a bad sign. Also check for leaks around any of the hoses, fittings, or plugs.
3. Bearings and ball joints - check for looseness, check axle and swingarm for play, check a-arms for play.
4. Tie Rods, steering stem, handlebars - check for bends that could indicate a wreck or roll-over.
5. Frame. Set the quad up on the rear grab bar and check for any frame damage, bends, cracks, or structural rust. Check subframe for bends and damage (common if someone wheelies a lot)
6. Air Filter - check if it is clean and maintained. A dirty unmaintained filter is not a good sign.
7. Chain and sprockets - Check for wear. Sprocket teeth should not be thin or hook shaped. Check the o-rings on the chain for splitting or cracking.
8. Tire wear - are the tires original, replaced, or are they bald? Can indicate the amount of hours the quad has seen.
9. Electrical - make sure all the lights and switches work. If you have a volt meter, check to make sure that the battery is charging.
10. Controls - make sure the all the controls work...throttle, brakes, etc...
11. Body condition - cracks or stress marks could indicate a roll-over or wreck.
12. Oil cooler condition - fins in good condition, not bent or clogged.
Check for blue smoke on start up. This could mean leaking valve seals or bad rings. Leaky valve seals usually smoke more on deceleration.
The 400ex may have a bit of valve noise which is normal. At most it should only sound like light clicking, but not loud rattling.
Many 400ex's also have a subtle whine at idle that to me sounds similar to the compressor whine from a supercharger. This is also normal.
Check all of the intake boots for cracks and make sure the clamps are tight.
Take any "low hours" claims with a grain of salt. Everyone tends to advertise their quads as "low hours". Condition and maintenance will tell the tale.
Thanks for the advice. I ended up buying it yesterday. Checked everything over and passed my inspection. Couldn't beat it, picked up an 07 for $1800. Only thing that it really needs is new plastics because the original owner cut the fronts for the race look. Got home yesterday and rode it for 2 1/2 hours and it never skipped a beat. Engine was dry (no leaks) and it was tuned right.
One thing to check into on those is the recall that Honda had on them. I am not sure if it was still there in the 07 models, as it started in the 04 or 05 models, whichever they put the reverse on first. Anyway, a spring in the switch would break or something and cause it to act like the reverse switch was actuated, which means if you accidentally stepped down on the shifter it could go into reverse and cause it to lock up and everything. Just a heads up. Other than that, the EX's are awesome and dependable machines.