When you make a payment of U.S. source income to a person or entity that you know is considered a foreign person for tax purposes, you are normally required to withhold U.S. income tax at a rate of 30% and report it on Form 1042-S. You may also need to withhold this tax when you pay a U.S. person acting as an agent of a foreign person. And, you may need to identify the actual beneficial owner when you make payment to a flow-through entity or a foreign intermediary.
You can check the withholding tax status of a foreign person by reviewing the applicable Form W-8 and the corresponding withholding statements and documentation. The fact that you may be held personally liable for the tax and any associated penalties and interest if the tax obligation is not met, makes it important to be clear about who you are paying and how the associated U.S. income tax obligation is being met.