Employer- Employee Relationship evidence

Hi All,

I am in India having H1b stamped from Company A although I haven’t traveled to US till now. Now, I got a call from Company B to transfer ( New process - Quota Exempt) to them and go & work in there US office. My skillset exactly matches with the role they are offering. Company B filed my H1b -premium processing and that got approved . I have valid H1b petition for Company B. Now, for the peace of mind I am planning to go for fresh stamping although I have one with Company A.

Till Now I have not joined Company B and will join only in case I have to travel to US.

My question is what proof I can submit as Employer-Employee relationship?

will Offer letter work in this case?

What all other documents can be asked by VO.