Generally speaking, courts require what is known as a "Due Diligence" affidavit to show the efforts that you have made to locate a person.    That can be talking with friends, relatives, last known employer, looking the person up in the telephone directory, etc.  In this day and age an internet search ("facebook") is also a good way to search.  If the court is satisfied then they will let you give noticee maybe by publication and then allow you to distribute the money in to an account so that you may close out the estate.  Good luck.