In general, the person who named you as a beneficiary will notify you that you are listed as such. If you believe you are the beneficiary and were not notified, contact the insurance company or the executor of the estate. You will be required to present a death certificate for the insured person and to prove your identity as the named recipient.
If you do not know whether a policy exists, the first place to check is the deceased safe deposit box or other location where important were stored. If that fails to turn up any results, look through the person's banking records for an indication of policy premiums paid out to insurance companies. Finally, the insurance company may not be aware that a policy is ready to be paid until the payments fall into default, at which time they will attempt to contact the insured person and then that person's named beneficiaries. When a policy falls into arrears is typically the first sign that a life insurance policy has come due, and watching the mailbox of the deceased may reveal the company you need to contact.
Insurance companies will take reasonable steps to locate beneficiaries, but if that person is not found the policy will eventually revert to the state. The state will hold unclaimed policies and other financial assets for 5 to 7 years, and can be located by searching through unpaid funds databases in the state where the deceased person lived. You will need to know the full name of the insured person and their last home address, among other information.