Video and audio. How reliable are they? How hard is it to tamper with either one? How do you KNOW that what you see/hear really happened?

I have no background at all in falsifying anything but I expect others do. Is it extremely difficult to sabotage what you see(video) and what you hear(tape recordings et al) without it being detected? Could YOU differentiate between a 100% truthful recording of what transpired and one that was a phony? How?