Why do you think that your mother has Munchausens? Is she harming you or a sibling intentionally? If so, call the police, talk to a trusted adult, if your at the hospital, talk with any of the doctors or nurses and tell them your suspicions.
What should I do if I think my mom has munchausen syndrome by proxy?
Not a thing. Leave any diagnosis to the professionals. Attempting one yourself, based on a TV soap or the Internet just doesn't work in real life.