To the best of my knowledge you would have to call the insurance company and give them the policy number if you have it. If you don't have it you would probably need to give whatever information they ask for, so they can look it up. They may not be willing to give information over the phone, in which case they may tell you to request it in writing.
Once you've established that you are who you say you are, and what the policy number is, you should then be able to ask who the beneficiary is.
Regardless of what the actual procedure is, the first place to start is by calling the company and asking what you need to do from there.