I remember running SETI@home on my computer back in college. While some of the enthusiasm for this project has waned since then, people in the field of SETI, or the Search for Extraterrestrial Intelligence, are still hoping and searching for evidence of aliens. And once that happens, we'll be ready.
It turns out that any evidence of extraterrestrial intelligence can actually be quantified on a spectrum, known as the Rio Scale. This scale was developed by the astronomers Iván Almár and Jill Tarter in order to quantify how important and earth-shattering a discovery is (or will be). The scale ranges from 0, for no importance whatsoever, to 10, for extraordinary importance. It is actually a mathematically defined scale, with a number of inputs that include how credible the report is, and the nature of the evidence of extraterrestrial intelligence. If you are interested in examining the rating for a scenario of your imagination, may I direct you to the Rio Scale Calculator.
As we actually have not yet had such discoveries and if your imagination is at a loss, Iván Almár and another astronomer Seth Shostak have written a paper that tests the scale by grading alien movies according to this scale. This paper is, in a word, amazing. For example, the aliens in Independence Day rate a clear 10, while the monolith in 2001: A Space Odyssey merits only a 6. Please note that the paper is rife with spoilers, as might be expected.
But while the Rio Scale is a lot of fun to play with and think about, it actually raises an important point. As Almár and Shostak note:
So go off and use the Rio Scale, recognizing that alien encounters are probably going to be a lot more ambiguous than we realize. But we can now measure that ambiguity.