You kind of have to treat it as a kind of weird video game. If I remember rightly they have one actual hazard that they mark in each clip, and you are meant to click as soon as you see it. It was something like 5 points if you clicked almost immediately and then decreases after that. They have those point blocks along the timeline of the video, so if you click whilst within that block, you score that number of points.
Just found this picture which illustrates what I'm trying to say - you can see the video timeline and the blocks I'm talking about at the bottom (which of course you don't see in the actual test):
As for actual tips, the best thing to do is keep practicing. In the actual thing they tend to make the hazards pretty obvious (if it starts with going along a country-lane, just clicking as soon as you see anything usually works).
General rule is only click when you see something where you'd have to take action in real-life (slow-down, brake, turn etc).