How would you possibly know someone was both taking Medicaid and was doing so illegally? Unless you have pretty detailed knowledge of a person's life and their family, I don't see how you would know that. It seems to me that the vast majority of Medicare fraud is from health-care providers (or alleged health-care providers), medical equipment suppliers, and organized crime.
So, if it is ordinary people, and you are simply suspicious, I'd say leave them alone. If it is a 'health-care provider' and they speak Russian, well then I'd say report them at your own risk.