1. Try to look for a camera from amateur or single owner. Avoid a professional owned camera, as most probably it has gone through heavy use in a short period.

2. Is there any scratch, paint lost, brassing, dent on the body? Try to avoid body with dents. 

3. Is the shutter curtain ok? (no fingerprint on it, smooth operation for every speed)

4. Is there any dust, mould inside the body? (check the mirror and look through the viewfinder (near and far) when checking). Go to a bright place, if the shop is not bright enough, ask the permission to bring the camera outside the shop. 

5. Check the battery compartment, is there any sign of battery leakage? Avoid if the camera has battery leakage history. Reason is because battery leakage means acid of the battery might have gone into the camera and damaged the circuit board. It might still be working at the time of testing, but it will definitely corrupted the circuit board after some time.

6. Check every shutter speed, is it working? Try listen to the different in speed especially for the lower 1/30s and 1/250s, there should be obvious difference.

7a. For film camera, check the film sproket, if many films have run through it, there should be sign of use. check the foam (mirror damping and camera back), are they new? Improper storing or aging might detroit the foam. However, it costs little to change the foam.

7b. For digital SLR, try some software like Opanda Exif to check for the shutter count of the DSLR. Some old DSLR has no shutter counted recorded (eg. Nikon D1, Canon D30, D60, 10D). Newer DSLRs have shutter count in the Exif info and can be checked using the Opanda software (freeware). Most amateur owned DSLR will have less than 10,000 shutter count. Nikon DSLR has around 100,000 shutter count life for D70, Canon DSLR has around 50,000 shutter count life for 20D. Canon 30D is said to have around 100,000 shutter count life. Professional series DSLR will have at least 100,000 shutter count life.

8. For an AF body, get more information regarding the operation of the body before testing out an used one. That's because you have to check all the functions of the body and make sure that they are working fine. With modern DSLR, user is advised to know the features before hand as it is especially complicated for a beginner to understand the functionality of a DSLR on the spot, needless to say testing it.

Never hurry yourself in making a decision, do some study regarding what you really want and take your time to look around for the best bargain.

Hope it helps, happy shopping!