You can check the specifications of your phone in the official webpage of your phone manufacturer. In addition, you could check the specifications of your phone in brochures from authorized stores... or you can just ask the sales representative about the phone (if the sales representative knows enough about your phone). Any (if not all) of these can be done prior to purchasing a phone.
Thing is, the magnetic thingy is probably not one of the things people look for in a the specsheet when shopping for a new phone... GPS is now already taken for granted that a phone has to have.