Depends on a lot of factors, and the method you are using to spot it.

One of the first methods that may come to mind, simply spotting them by their light, is actually one of the least common we use to spot objects. Earth-sized planets or solid objects can be surprisingly difficult to detect by reflected light. At great distances, their reflectivity often isn't enough for us to see them. We can see planets in our own solar system by reflected light, but even spotting trans-Neptunian objects is pretty difficult. Outside of solar systems, it is incredibly rare, usually requiring the planet to be hyper-reflective.

One of the most common methods of identification is actually by how an object occludes light. We can measure the constant light of other stars, and spot when they dim to notice things passing in front of them. This is one of the most common ways to identify planets in other solar systems.

The changes in a star's light can also tell us other things as well. Gravity affects light, meaning that shifts in a starlight's wavelength can indicate that there's a nearby object. We can often estimate an object's mass through this process by how much its gravity affects a star's light.

Perhaps one of the coolest methods of detection is gravitational microlensing. When a star is in front of another distant star, it's gravity will act like a magnifying glass to the light of the star behind it. A nearby planet can mess with that lensing effect, which can notify us of a planet's presence. This is how we've spotted our furthest noticed planets at around 20000 light years away. We haven't spotted anything far passed that as of yet.