Some Doomies could see events, others had feelings of what would shortly occur, including events of doom, hence the name.
A Doomie was one who could see the future; this ability manifested in degrees from flashes of danger to full-blown visions. In Pilgrimage to Hell, weaker doomies were called "sensers"; later books make no distinction, calling all muties who can sense the future "doomies". Full-fledged doomies are very rare; most people with this power (like Krysty Wroth) have a "danger sense", but not visions.
A Sensor was a mutant who could sense danger, sort of a low powered doomie.