Open Question: How do you determine all the creditors of a deceased relative?

During my father’s life I was occasionally contacted by alleged creditors, but I did not retain their contact information or their letters. He passed away recently, and under small estate probate in NY, you need to list his liabilities. How am I to determine to whom he has outstanding debts since none of those alleged creditors have sent me any current letters w/in the 2 months of his passing? Do I need to wait longer to see if I get new letters? His estate is only $1700 and his funeral expenses far exceeded that.