If you are certain that your father had a life insurance policy but cannot find the policy among his possessions, there are a few ways to find out which company he had the policy with. Even if you still can't find the policy, be patient, because the insurance company he had the policy with will most likely attempt to get into contact with him after a premium or two has been missed.
In many cases, the first clue an insurance company has of a life insurance policy coming due for payment is when the policyholder stops making premium payments. After a premium or two has been missed, the insurance company will first attempt to contact the policy holder, and, failing that, they will next attempt to contact the beneficiaries or other contacts listed in the policy.
In the meantime, check with any banking institutions your father dealt with, looking for insurance companies that have been receiving regular payments. You can start by simply looking through your dad's old checkbooks, and then by contacting the customer relations department of the banks you suspect he had accounts with. You may discover that locating his insurance company is easier than you had thought.
If your dad dealt with a CPA or other accounting firm regularly, they may have a record of his insurance policies as a matter of course. No matter which company he bought the policy through, there will be a financial paper trail that can be followed, making his financial institutions and accountants your first and most likely method of finding out details about the policy and who issued it.
It is not a requirement of the insurance company that you are able to present the actual policy to collect on it. By verifying that you are his next of kin and presenting the insurance company with a copy of the death certificate, you can initiate the settlement process. Finding the insurer is generally more difficult than putting the payout process into motion.