pawn shops can sometimes yeild some decent lenses, and you can easily inspect them. Here are some basic things to check out when evaluating used lenses:
1. visual inspection: check for any obvious signs of dropping, scratches on the body of the lens as well as the optics (both front and rear). gently shake the lens to be sure it doesn't rattle and feels tight. check mounting surface and be sure the contacts are in good shape, etc. Some Nikkor lenses have plastic mounts, while the 'higher' quality ones will be metal.
2. pop off the two caps and look through the lens: check the optics as well as the aperature blades for alignment, etc.
3. if the lens has an aperature ring, look through the lens and adjust it from max/min and check for smooth operation throughout the range and be sure watch the aperature acutally get bigger and smaller. It'll be obvious.
4. check the 'action' on the focus ring, too. be sure it's smooth and not too tight or loose throughout it's range also.
5. if you can...put it on your camera to check it out for quality (front/back focus issues, chromatic abberations, sharpness, etc.). also listen to the lens as you fire the shutter, it should not sound 'weird' (whatever THAT means.
I like Dave's theory of getting an aftermarket or less expensive lens for wheelin' camping, etc. and if photography is important to you, invest in really nice glass when you want snap those "keepers". it truly is alll about the glass.