Educated guessing is a good way of summarizing it. You can never be 100% sure of an outcome (unless as you say you have insider info). However, by following the news and key events likely to impact the price of bitcoin (regulation, past issues that have affected the price) you can then work with a likely outcome. People who trade for a living (whether in bitcoin or fiat) develop a keen sense for picking up signals from the market to make these "educated guesses".