How are rogue planets discovered? Planets are usually found by observing a star and waiting for the light level to drop when a planet passes in front of it, but what about rogue planets that don't have host stars?
The most successful methods are by gravitational microlensing and direct observation in the infrared or far infrared.

The transit method doesn't work well for rogue planets, because usually at least three transits are needed to confirm a planet.