The easiest way is to try to look for confirmation or a denial of what you just saw. But in the given example all you have is a few minutes of video footage which doesn't give you any context. So in a documentary, you would expect those people rejecting water to explain why they are rejecting it, while in that video all you have is the images without any context. That's obvious, I'd say. Now, given this, you would be surprised by what people might say they are doing what they are doing, as shocking and uncomprehensible as it might seem in a video.

For example, I was having a lot of questions about the situation in Calais, France, why would there be people looking to risk their lifes to go from France to England? Seeing a couple of documentaries on this situations with interviews with some of the people there, I got a completely new insight into what's happening there.