Taking over tax prep work for an S-Corp with two shareholders.  Shareholders want to take year-end distributions from the corporation.  Where can I find the shareholder's stock basis is the s-corp to make sure they don't distribute more than their basis and end up with a taxable distribution?

Distributions were taken last year and the box is checked on the Schedule E stating that the basis calculation must be attached, but the prior CPA did not actually calculate or attach the shareholder's basis.  Where else might I obtain this information?