Simply by finding their location....you can aim your receiver units at them and wait for “a signal” of any type. You can figure that sooner or later, you just might pick up a sign of some sort.
I won’t sit and ponder on the odds...but you have to sit and wonder what happens when we finally do find one planet with intelligent life. At that point, you can bet odds of hundreds of other planets with such circumstances. So we are merely waiting for the one opportunity to arrive.
I won’t sit and ponder on the odds...
Because if you did, you would realize its impossible.