If your name was on the check (as in you are a named beneficiary of the policy) the conservator has no right to retain the check, as far as I am aware.
If your name (or another beneficiary's name) is not on the check (as in there is no named beneficiary) then the check is payable to the estate estate and the conservator does have the right to deposit the check into the estate's bank account in order to settle any debts or taxes owed by the estate, or cover funeral expenses. Settling an estate can take around a year, or longer depending on complexity. Whatever remains would then be distributed according to the will. If your father died intestate (without a will) then that's a whole other very messy can of worms.
Federal and state laws also strictly dictate what the conservator must do with the estate's assets and any insurance checks made payable to the estate -- or payroll checks made to your father, etc -- so he may not have that much control over the situation either.
Although honesty is the best policy, the conservator's fiduciary and legal obligations are to the estate, not to you. So, technically, he can tell you anything he wants (unless that check had your name on it... which would make it stealing) to keep you out of his hair.
I'm not an attorney... this is just what I know from experience... so you should definitely contact an estate attorney for more accurate guidance about how to handle your situation.
Answered Feb 07, 2013
Edited Feb 07, 2013