1.Without having any idea it is very difficult to unearth the details of the immovable properties your husband acquired.
2.Searching of registration offices may yield to some result but again it depends on the city where the proeprty is situated.
3. If you are expecting such properties to be situated in USA then you will have to take help of American legal professional.
4. You can talk to your mother in law on this issue and make a mild warning of legal proceeding if she does not disclose all the informations.
5. You may know that on death of your husband both you, your mother in law and children if any jointly inherit all his properties in equal share.