Montessori is not a trademarked or copyrighted name so anyone can call themselves Montessori even if their teachers are not trained.
For true Montessori schools, find one affiliated with the one of the Montessori Accrediting organizations.
Association Montessori Internationale
or
Home | American Montessori Society
Note that some schools that are not accredited do follow the Montessori curriculum and use Montessori materials, but they may have teachers who are not certified in the method from the organizations above. They also may add other things to their curriculum.