How do you prove to the IRS you were an S corp?

How do you prove to the IRS you were an S corp?
A corporation must elect to be treated as an S corporation by filing Form 2553 before or by the 15th day of the third month of its first effective tax year. The IRS can and often does allow late election for "reasonable cause," which can include not knowing about or understanding the election requirements. All shareholders must consent in writing to the retroactive election and amend their returns if necessary to reflect the change.