Bicycle Mechanics - What to look for when buying used bike?

A co-worker is wanting to take the plunge and get into cycling (yay!), but since he's on a limited budget he's looking to buy a used bike. He found a deal on a used 2002 Trek 1000, and since the seller is local he will be able to ride it and check it over before buying. I offered to go along and check it out with him, and I'm wondering what exactly to look for and check out when looking at a used rig. Any suggestions?

dents and cracks (take your time they are hard to spot), de hanger if at all out of line, true and round wheels check spokes to see that nipples havent been threaded down all the way (thus allowing u to true them up as needed) just take your time and make sure it will fit them!!!!!

First and foremost: Does the bike fit him????

The most important place to look for cracks is the joints where the frame tubes were welded or brazed together.

Run a finger along the tubes and feel for UNUSUAL bulges, etc. It's amazing how something can be hidden from the eye but not your touch.

PJ

Besides the obvious (round wheels, and a good fit)...
Frontal collision will sometimes leave a very small visible effect, just some ripples in the paintwork behind the head-tube. I have one bike I've done that to, and I wouldn't buy it from me.
Check that the cranks are not bent inwards from a fall.
You may need to budget for replacement transmission if the cogs show sharks-tooth profiles.

Check all the moving parts, I think you are more likley to find a bike that has been badly maintained or sat outside in the rain than crashed and replacing parts can quickly get more expensive than just buying new.

I concur with what I have been reading in this thread. Since one generally buys a bike for its frame, start the inspection there. The frame must fit your friend comfortably and must show no signs of abuse, collision damage, or cracking. Next, check the condition of the wheels, particularly the rims, spokes, and bearings, because these represent a large percentage of the value of the bike. Make sure the cranks, pedals, and chainrings run straight and true. Measure the chain -- it is time for a new one when 24 links exceed 12-1/16". Good luck to your friend, and thank you for promoting the world's greatest sport.

A few other points that weren't mentionned so far:

- Wheels : better with aluminium rims, as steel rims don't brake well when wet.

- Rust and other signs of wear. Don't worry too much about superficial rust, small paint chips and the like, but something much more substancial could be a sign of neglect.

- Gears. Make sure the bike has decent gears. Newcomers shouldn't have "racing" gears, but relatively low gears.

Finallyput aside a budget for a tune-up, part replacement and the like. Depending on what you want to do and what you want done by the LBS, and on the mileage he plans to do, brake and shifting systems should be checked and might need new cables, new housings, new brake pads, etc. And if the bike hasn't been used or maintained for a few years, wheels and bottom bracket might need grease. Still, even if you add up the cost of a tune-up ($50 to 100 depending on what needs to be done, much less if you or he do a few things), it could well be a bargain.