No guarantees.

It's like closely observing a relative and how they emotionally react to certain situations. No matter how much time you put into observing them and taking note of how they react, you can't ever be 100% sure how they'll react in a similar situation in the future, but you'll probably be right more often than wrong.

In the same way, collections of traders and investors usually react in the same way that they have in the past to similar market conditions. Not always, but more than 50% of the time, which is good enough to make profits from.