When buying a used bike...

[Hoochie Papa]

Since most people won't let you test ride their bike, your only option is to look it over and sit on it. Once I am looking at one, what exactly should I look for? Is there anyway to tell if the fork seals are weak?
Looking for oil leaks is easy enough, but they are easy enough to hide too.
What should I look for in the suspension? Is it a good idea to ask the seller to drive it around the block once?

[Papa Lazarou]

Leave the seller a wodge of cash (usually the asking price) and test ride it. If s/he refuses, walk away. Always meet a seller at their home.

Fork seals are pretty easy to fix, so not a major prob if they're shot. Usual signs are a bit of oil on the exposed fork tube. Mind you, I oil the exposed fork tube to keep stone chip damage (and subsequent rust) down-and the seals are fine.

Rear shock-a ride should tell you. Otherwise, push the back of the bike down to check there's no graunching noises, that there is damping and that the spring is working. You can usually reckon on a budget shock being in need of repair or replacement after 15-20K. Read up on the model of bike before looking at it to check suspension quality.

[Rabidsnipe]

http://motorcycles.about.com/od/buyingamotorcycle/ss/how_to_inspect.htm
http://motorcycles.about.com/od/buyingamotorcycle/ss/test_ride.htm
http://www.wikihow.com/Buy-a-Used-Motorcycle

[UHOH]

...Is there anyway to tell if the fork seals are weak?

grab front brake and pump the forks up and down a few times, then feel/look for fluid that gets past the seals.

also, look at how the seller sees you.  In my area many people would let a responsible rider test ride one ...

[jsanford]

This is The Bible for looking at used bikes, and the author recently added a checklist suitable for taking to a seller's house:

http://www.clarity.net/~adam/buying-bike.html

 

[Rabidsnipe]

This is The Bible for looking at used bikes, and the author recently added a checklist suitable for taking to a seller's house:

http://www.clarity.net/~adam/buying-bike.html

 

Wow!  What a great read. That should be mandatory reading for any bike owner.