Its not about knowing for sure, technically there is no such thing as sure thing for anything in universe, however that doesnt mean that one cannot execute process with positive outcome over long run just becouse the outcome is never 100% sure. Therefore it is important to have an edge where one is able to find pattern that has 70-80% statistical reliability and execute based on that, it is totally doable executing such pattern with positive outcome / profits even if it is not 100% guarantee that bottom will trully be formed. This is core basis of any profitable strategy.