The only way to know is to go back to the beginning of time and create a basis worksheet for each year.

https://www.irs.gov/pub/irs-pdf/i1065sk1.pdf

PDF page 3 (or just google, you can find plenty online or use the IRS one to make your own spreadsheet.)

In a perfect world the partner has all of this information.  Here in the real world you might get lucky and the partnership accountant can get you most of the information.