You look human to me...

But how can I really tell?  Well, the squiggly letters on the comment box are how we currently do it.  Its called CAPTCHA.

But the challenge is increasing.

Herein lies the key to leaving squiggly letters behind. As Alan Turing laid out in the 1950 paper that postulated his test, the goal is to determine whether a computer can behave like a human, not perform tasks that a human can. The reason CAPTCHAs have a term limit is that they measure ability, not behavior. The history of computing shows us that machines will eventually learn how to perform all manner of tasks—like identifying words, for instance—that we currently assume only humans can solve.
How might it be possible to measure behavior rather than ability? The other day, I was writing a note to company using the online form they provided for media requests, doing the usual amount of typing, backspacing, and retyping as I tried to phrase my note in a way that would make them respond quickly. It occurred to me that the random, circuitous way that people interact with Web pages—the scrolling and highlighting and typing and retyping—would be very difficult for a bot to mimic. A system that could capture the way humans interact with forms algorithmically could eventually relieve humans of the need to prove anything altogether.

Any solution that could replace CAPTCHAs en masse would have to be free, work across a wide variety of platforms, and be easy for the average blogger or Web admin to install. One of the reasons that CAPTCHAs have spread like kudzu, I suspect, is that they're so easy to implement—in some cases, as simple as checking a box on a site that helps you set up an input form. The more a bot-fighting algorithm can insinuate itself behind the scenes, the better. In the meantime, we'll all have to keep debating the eternal question: Is that a W, or is it a V and an I attached at the hip? [More]

Of course, we may discover with more sophisticated "humanity" tests there really are aliens among us.

Speaking of which, have you noticed now everyone is carrying a cell phone that takes pictures the number of UFO reports has plummeted?