Reverse SETI; Or how easily could aliens know we are here?

I've been wondering about how easy it would be for some alien or aliens detecting that "intelligent" lifeforms exist in the solar system. Essentially, how easy or how hard would it be for aliens to know with a reasonable degree of certainty that we are here. With the limits of our knowledge, what methods could they use? Which would be likely to produce reasonable proof? What would be any telltale signs of "intelligent life" that the aliens could point to as evidence?



If they had an alien analog of SETI, how useful would it be?

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