Obviously you do not have a copy of the policy. I'm wondering how you know a policy exists.
Just some ideas.
1. Go through all Mom's paperwork looking for a policy.
2. Go through all Mom's old checkbooks looking for a canceled check where she paid an insurance premium. Perhaps even talk to Mom's bank about getting a list of all paid checks for the last couple of years.
3. Call the State Insurance Commissioner. Ask for a listing of all life insurance companies licensed to sell insurance in your state. Explain the problem and ask if they have any idea other than you writing letters to every company.
4. Hire a psychic.
If none of those work and no claim is filed, eventually the insurance company will turn over the proceeds of the policy to your state, most likely the State Treasurer. The Treasurer will annually produce a list of "unclaimed property". Monitor that list every year in hopes you will see your mom's name. Just be advised it takes anywhere from 5 to 10 years for the funds to escheat to the state.