Generally, the Tax Authorities want to see the form 6166 or form 8802 with Proof of Payment as Proof of Tax Liability for businesses who are based on the US. 

An example of how these forms look like can be found below. 

Please let it be known that an empty form 6166 or an empty form 8802 have no value for the registration. We ask you to please provide these documents pre-filled in and issued officially by the IRS.

Form 8802:

Form 6166