Check the business’s tax documents, W-2s, 1099s, or prior IRS correspondence
Ask the business owner, accountant, or payroll provider
Search the company’s website, especially the investor relations or legal pages
Review state business registration records
Look up SEC filings if the business is publicly traded
Use paid business information databases
Contact the IRS Business and Specialty Tax Line if you are authorized to obtain it
Request a W-9 from the business
Check loan, banking, or vendor onboarding documents if you have access
Search local business licenses or permit records