Both of these web sites may charge you for their services. I believe the cost of the M.I.B. service is $75. I think the fee for the findyourpolicy service is $19.95.
You can also check with your State's Unclaimed Property Department, in the event that the life insurance policy was unclaimed.
If you have access to the insured's bank records, look for cancelled checks that were paid regularly to an life insurance company.
However, if the life insurance policies were term policies, they may have already expired before the insured passed away, or they may have lapsed in coverage due to non-payment of premiums.