How can I be sure if I suspect someone has munchausen syndrome by proxy?

Not easy. It is often a difficult diagnosis to make - this should be done with the guidance of the treating physician for the patient (usually child) in question. It is a diagnosis of exclusion, meaning that all other relevant options have to be ruled out.
Call child svcs. If you have reason to suspect this pattern you need to call the child& family services division of your state. Most states have laws that keep your report confidential and shield you from liability if any issues come up later if the report is made in good faith. Their case workers can make the appropriate calls & eventually arrange for a diagnostic evaluation at a treatment center.