When raising capital at what point is it required by United States law that the investor be accredited, and if possible what are the laws stating these guidelines? Also, what due diligence is required to show that the investor claimed to be accredited, and when does the claim expire?
When you are raising money through a private placement memorandum you need to qualify the investor. This qualification process should be documented in a "Investor Questionnaire". Completion of the Investor Questionnaire comes prior to presenting the PPM. The investor questionnaire includes documentation of their qualification as an accredited investor.
Under your state's Blue Sky laws you may also have an option to have a certain number of unaccredited investors participate in your offering. Check with the state regulations to understand that implication.
Where you work with a lawyer, a licensed broker, an investment adviser or a specialized marketing company to support this effort-- they will have the documents that you need to do this in a way which does not unnecessarily expose you and your company to unneeded risk.