Identification on pain pills is required by the Food and Drug Administration (FDA). According to Pharmer.org "All FDA-approved pharmaceuticals must have a unique imprint. An imprint is any marking on the pill, such as an engraved logo, numbers and/or letters or a raised or debossed symbol." However, there are generic manufacturers that still allow some of their pills to be unmarked, despite the law. There are ways to identify these generic pills even without an imprint identification marking.
Search a website that specifies in pill identification. One such website is Pharmer.org. This pill identification site allows you to scroll through the possible matches to the generic pain pill you are looking for. The benefit of this website is that it gives pictures of the pills to make it easy for you to search through them.
Ask your pharmacist. Take your generic pain pill to your local pharmacist. Pharmacists are experienced in identifying numerous pills. Letting the pharmacist know that it is a pain pill will narrow down their search for your answer. If it is your pill, let her know of some of your ailments, which may further narrow down the possibilities.
Ask your members of your household if they are aware of what it may be. It is possible that someone you live with knows what that particular generic pain pill is. The pill may be his and it was dropped from their container when he went to take one. Have him verify that it is exactly the same as the ones he has in his pain pill container.