How can I know if a video has been manipulated/staged/faked? Recently, a video has been posted about a politician named Varoufakis sticking his middle finger to Germany.

Later, a German TV host admitted faking Varoufakis 'middle finger' video on YouTube, showing some smart germans editing it using a software.

As a skeptic person, I'm not sure which one is the real one (statements of authority, videos uploaded do not really make me believe anything).

I only believe in evidence.

How can I know or basically prove (or be 99.999%) sure that a video is faked or staged using a software? What would be the methods or procedures? I would appreciate if a video professional would help me.
Authenticity in video is dead.  It's been dead in photography for a long time.  It's just something we're all going to have to get used to.  Professional forensic analysis might sometimes disprove a video's authenticity, but it will never be able to conclusively prove it.