So how are you going to evaluate then? Draw straws?
I don't think that definitive proof of drafting ability exists, and I mostly base my feelings of the job done on whether or not their assessments/reasoning/choices seem shrewd, lucky, or unlucky, with the benefit of hindsight. That said, consistently impressive or unimpressive results over a prolonged period of time can be pretty telling and difficult to argue with, because it's unlikely for good or bad luck to account for that.
With the current group, it's kind of all over the map at the moment. Virtanen and Juolevi were drafted based on pretty questionable reasoning, IMO-- Even if they happened to turn out great, I'd find the decision questionable and the outcome lucky. Pettersson and Hughes were drafted based on sensible reasoning, IMO-- Even if they happened to bust or ran into career-ending injuries, I'd find the decision worthy of support and the outcome unlucky.
I'm unaware of the conditions of the Boeser pick, but if hypothetically his career falls off a cliff because of the back injury, I'm certainly not going to hold it against the choice to draft him. That'd be pretty stupid.
It's like judging the abilities of a poker player based solely on how much they win after a few outings, rather than their actual strategy. You might expect their winnings to reflect their abilities long term, but to evaluate their winnings on a game by game basis and point to that as a direct measure of their skill is foolish.