I want to sell a property I inherited from my parent 50 years ago i before I immigrated to usa. How to provide the IRS with a fair market value of the property at time of inheritance?

That will be tricky. I would go to my library, ask if they have any want ads from50 years ago, probably micro film or microfiche, and peruse them. As an alternative, check with your local tax office to see if they have a valuation from 50 years ago.