Campfires are visible from orbit with no special magnification, so any time since humans learned to control fire, a visitor making an orbital survey could have pinpointed likely outbreaks of sapients. They'd have been recognizable as either a) pinpoint fires that don't spread even though there are combustibles in the area; or b) artificial fires in places like the desert where there are few combustibles.

Earlier than that, I'd have to go speculative and think about sensor technology capable of distinguishing the activities of sentients. Present day optical technology can resolve down to the centimeter range (so we're led to believe about military reconaissance sats), so perhaps traveling bands of hominids could be spotted from orbit and any artifacts they were carrying could be distinguished. Early humans stampeding herds of plains animals toward a cliff might also be easily recognizable as such.

I'm a little skeptical that visitors who actually wanted to find intelligent or potentially-intelligent species would limit themselves to EM emissions as the binary sign of intelligence. More anthropomorphizing, perhaps, and depends on degree of motivation.

In terms of surveying the solar system, remember that in recent decades we've started doing just that. Anomalists have discredited the idea of a formal search with lots of public fanfare, but I wouldn't think it out of the question that serious scientists keep the possibility in the backs of their minds as they pore over the results. If you're an astrobiologist hot on the trail of life signs on Europa or Ganymede, say, might you not wonder if it got started from discarded fast-food wrappers from some past visit?

I can't argue that either scenario wouldn't be a difficult proposition, just that perhaps it's not so improbable as all that.