How do you perform a background check on a business?

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How do you perform a background check on a business before you invest with them? I buy a D&B report on the company, check with the BBB, check references, check with the state to make sure that they are a legal entity and have a business licence. If they are offering an investment opportunity then I check to see if they're registered with the SEC. I will check with REIA members to see if anyone knows about the company. What else can you do check out a company?