1) Check for good gate action: The open-gate strength of carabiners is roughly 1/3 of the closed-gate strength. If a biner has a gate that rubs or sticks open, it should be cleaned and lubed. If this does not improve gate action, the biner should be retired. The same holds true for any gate locking mechanism.
2) Check for excessive wear: If you can feel that the rope-bearing surfaces of the biner are significantly worn (wearing off the anodization is normal after a few uses) the biner should be retired.
3) Check for deformation: If a biner has been loaded such that the body or nose has deformed—or the carabiner gate rivets have been bent (this usually results in poor gate action)—the biner should be retired.
4) Check for nicks or deep scratches: If a biner has nicks or deep gouges beyond the normal light scratching that occurs in use, it should be retired. Carabiners are more susceptible to surface damage near the nose hook or within an inch of the bending radii of the body.
5) Has the carabiner been exposed to extreme heat? If a biner has been exposed to "extreme heat" (i.e. a fire) it should be retired and destroyed due to possible negative effects to the heat treatment the carabiner underwent when it was made.
6) Has the carabiner been exposed to harsh chemicals or excessive corrosion? If your carabiner has been exposed to aggressive chemicals (like battery acid, petroleum-based fuel, ect.) it's a good idea to retire the biner. Likewise, any corrosion beyond the normal thin gray/white oxidation layer that forms on exposed aluminum should be grounds for retirement, especially if it starts to affect gate action (see #1).
In closing: Keep in mind that only YOU know what your gear has been through. If your instincts tell you that the gear is dubious, retiring it is a good idea. Confidence in your equipment is not only key to climbing at your limit but helps you stay relaxed and having fun.