They can be of any sex and may normally take any role (dominant or submissive) in the scene, but while on duty their authority is absolute. If a DM orders a play scene to stop, it must be stopped immediately.
DMs are usually people with several years experience in the scene, and who have training both in BDSM safety practices and first aid. They often have a special uniform or hat, but there is no standard way of denoting a DM. It is best to ask when arriving at a party or event how to identify the DMs on duty.