ProtoAvatar, I think the point that Purple Buddha was making is that we have been looking for intelligent life for a very short time, relatively to the lifespan of the Milky Way. I mean, it's only been in the past 15 years or so we've even been able to detect other planets, which even then have generally been the superJovian type planets (though we have been detecting rocky planets now), so if that's as best as we can see right now, it's quite likely we don't even have the technology to even spot civilization in other systems. It may even be the case the only way to know for sure if there's civilization is to go to their system and learn 'first hand'.
Expanding Buddha's analogy, imagine if the cats were blind and could only find each other by literally bumping into each other, which at this scale, is probably a close analogy. Either that or the cat that represents humanity is both deaf and blind, and can't tell that the other cats can see us :P
, it souldn't be necessary to see the alien species' planet in order to confirm the the aliens' existence.
Listening for their EM communications should be more than enough. We listened since the 1960s - and so far, nothing. The stars are strangely quiet.
To be fair - most EM signals we have so far emitted should be incomprehensible to any alien astronomer (due to interference from the sun). But the carrier wave could be identified as being artificial.
And, of course, we should be listening to EM signals from civilizations much more advanced than our own - the assumtion being that a more efficient/undetectable for us means for transmitting information (faster than light, that is) does not exist (according to relativity, such a thing can't exist).
Another way to confirm the existance of alien spacefaring species is to detect one of their vessels/probes - at present, this is possible only if said probe is near/in our solar system. If these aliens use Von Neumann probes to explore the galaxy, the probability of a Von Neumann probe reaching our solar system is significant - a Von Neumann probe is a robot that multiplies itself when it reaches a solar system/a place with prime materials, energy; the newly constructed probes are sent to other systems where they will multiply etc; in other words, their numbers increase exponentially.
About Purple Buddha
's analogy - imagine there are millions of cats in the forest (as my expansion of the analogy indicates should be).
What is the chance of one stumbling onto another - blind or not? In my opinion, there is a really good chance of such a thing happening frequently.