Choose or design any special senses for your creature, such as
low-light vision,
darkvision, or
scent. If you're making a sense from scratch, simply decide what it senses, whether it has a range limit, and whether it's precise or imprecise. For example, a ferrofluid ooze has magnetic scent. This means it can sense large sources of magnetic metal within 100 feet, and the sense is precise—about as acute as human vision in daylight.