There are three major events on the NHL calendar that will demonstrate the skills of a general manager: the draft, free agency, and trade deadline.
Out of the three, free agency is arguably the most revealing. The draft and trade deadline are mostly confined by who is available and what you have to work with, so GMs can occasionally be put in a bad spot through no fault of their own. In free agency, that’s all gone. There’s around 100 UFAs available to choose from and a healthy amount of cap space (and if there isn’t, it’s usually indicative of previous bad free agency spending) to work with.