It's all about probabilities. I was anticipating the top in biotech, and was leveraging the hell out of the trade. But I got scared since the trend was still for the most part at it's climax, but at the same time trending higher. It's very scary to be short at the top of the market because you never truly know it's the top. And you also don't know how low the market will go after that as well.
So the answer is, you never truly know. That's the hard part about trading. But I learned to just focus on probabilities, and limit the risk so that I don't blow up but can still earn a decent return.