The basics like name, email, phone and social media handles.
Two of the following as proof of business:
Valid membership to a major design organization (e.g. ASID, IIDA, AIA, NKBA, IDC)
Business or Resale License - Signed resale certificates are required for each state where you seek tax exemption (US only)
Business card with your design profession
Interior Design Certification (e.g. NCIDQ, CCIDC)