I'm not entirely sure I follow you, but I'll take a stab at what I think I understand.

There's a technical answer to the question of how we can recognize communications: Information Theory. It's possible to analyze any arbitrary signal, having no knowledge of its origin, encoding, or its content, and measure its information content as inverse entropy--the amount of structure is the signal-to-noise ratio. Interpreting it and understanding it is an entirely different matter, but we already possess sufficient mathematical tools to recognize communication when we encounter it.

As for the contention that "we haven't made meaningful contact with any of" the millions of other species on Earth, I think everything depends on how you define "meaningful contact". Most humans live in proximity to members of one or more other species, and our interactions can't be completely meaningless. My favorite species is the domestic cat, and through years of association I think I've learned to communicate as well as can be expected with a species that doesn't particularly care to communicate unless it's in the mood. I've developed a sense of the scope and nature of the intelligence inside that brain the size of an almond (alien worldview, but gets a lot of mileage out of that little dab of neural tissue). I think that the reputation of cats as aloof is inaccurate, and that humans who make the effort to meet cats halfway and communicate their way are rewarded by a certain amount of emotional bonding.

If that isn't meaningful contact, you have some terribly high standards. Multiply my experience by the experiences of humans with dogs and our other pets, and experience with domestic animals in general, scientific attempts to communicate with other possible sentiences like dolphins and whales, and the huge body of anecdotal stories of interactions between humans and animals in the wild, even the belief of treehuggers that they communicate with plants, and I'd say humans have been having meaningful contacts with all the life around us since the beginning of time.

The Western/Christian belief that humans are separate from nature is entirely false. The truth is that we're so much an integral part of nature that we can't have avoided meaningful contact with those millions of other species.

Sometimes meaningful contact is fatal to humans, but that which hasn't killed us has made us stronger. Our meaningful contacts with millions of species are recorded in our genes.