If you want to know about how easily they can fake anything and pass it off as real, take a look at the new big bang theory episodes coming out.
One of the actors is on the fake ISS and is floating about in so called zero gravity, so the question is...is he really on the ISS or in a plane or in a studio among special effects.
Sure, you can pretty convincingly make a movie faking weightlessness, but I don't recall seeing the things described in this thread (like floating water or floating hair) and on the Big Bang Theory. I'm sure that given enough time and budget you can convincingly fake those things too, but probably not as a real time response made to an impartial request.
Personally I'm less interested in videos of people on ISS and more interested in what they do there.
For instance, consider the following: the company I work for has accelerometers in ISS that measure background vibration. On earth these show a magnitude of about 1g, while on the ISS they show close to nothing (I've seen the data firsthand). In order for this to be coming from anywhere on earth the data must be fabricated, meaning that they've tampered with the accelerometer unit itself or the devices that talk to it. That'd mean they at least understand a fair bit about how the system works. Bear in mind that we've actually received units back from flight, that not only show no signs of having been tampered with but also show that the accelerometers have experienced launch (bias shift due to vibration).
But okay, it'd still be conceivable that NASA could fake the accelerometer data somehow - they'd probably have to spend as much money doing it as they paid us to make the thing, but it's not impossible. So let's look at why the accelerometers are up there in the first place. It's not to prove that the ISS is really orbiting the earth, but to detect acceleration events. The reason why is because this information is important to the many experiments running on board the station; the primary reason to perform experiments on the ISS is to gain access to a (nearly) zero-g environment. It should therefore stand to reason that acceleration events would disrupt these experiments, so it's important that the experimenters have access to reports of such events so they can determine that the environment was bad instead of their experiment.
Now think about it a little further.. if these experiments are being done on the ISS in order to escape gravity that means it should be very obvious if the results all look the same as they did on earth. So NASA would have to be faking not just the results of our accelerometers but the results of everyone's experiments. And while NASA has access to how we design our equipment since we work for them, the same can't be said for everything on the ISS given that a lot of private companies have experiments running there. So NASA would have to be faking the experimental data recorded by a wide variety of sensors, applicable to a wide variety of fields, likely by modifying part of the internal design of the electronics. Not bad for an organization that allegedly doesn't even employ engineers. Sounds to me like the conspiracy would have to be known by a lot of technical workers afterall; that or it would have to extend to a lot of private organizations outside of NASA.