Here ya go.
The forms the most employers say ther are "required" to have are the W-4 and the I-9. The W-4 for withholding of Income Taxes and the I-9 for Verification of Employment Eligibility. I will give you just a snippet of the actual LAW, and then where to go to read the entire LAW.
I-9 - Code of Federal Regulations Title 8 Chapter I Section 274a.2 = "This section states the requirements...." "...limited to a person or entity who is either an agricultural association, agricultural employer, or farm labor contractor..." DOES THIS DESCRIBE THE EMPLOYER?
W-4 = Code of Federal Regulations Title 26 part 31 Section 31.3402(p)-1 = "An employee and his employer may enter into an agreement..." THIS MEANS YOU "MAY" OR "MAY NOT" USE THE W-4. (I do conceed that not using a W-4 is very hard to bypass)
Basically, the employer is only "required" to do acouple of things. Make sure the person is leagally able to work in the United States of America. This can be done by simply having the worker "show" proof of eligibility. If there isn't a legitimate reason to believe the person is lying with that proof, then it is ok to accept that proof "shown" to them. If the person is eligable, the employer is not "requred" to have anything else.