If you can't find the policy in your parents home and safe deposit boxes, the next step would be to check through your parents canceled checks and bank statements to look for evidence of payment for life insurance. Assuming you have the appropriate documents, you can order canceled checks and past statements from the bank. You may also want to do the same type of search through credit card statements. It would also be a good idea to contact your parents' employer to find out if there was life insurance made available through the employer.
There are also companies that help with this sort of thing. One, http://www.mibsolutions.com/lost-life-insurance/ assists insurance companies with fraud prevention and has records of life insurance policy applications made over the past 13 years. For a fee, they can search through life insurance applications they have on file for leads as to what company your parent may have applied to. Another, http://www.l-lifeinsurance.com/ has a large database of insurers. For a fee, they can send a letter to each insurer asking if your parent had a policy and if you are a beneficiary under the policy. If so, they will have the insurer contact you.