I look for good ideas and especially good people. They must have a legally registered business in the U.S., a business plan that demonstrates the financial solvency of the business, and are willing to sign a contract.
I would NEVER do business with less than these factors. Without them you do not have an investment at all. You have a promise from a kid on the internet. You need the legal backing to sue them if they scam you. Only a written contract is really useful in court.