It's a subjective measure of what you see with the eye-test. I guess it's different from everyone, which is why I'm not trying to infringe on your right to believe Bedard is generational. I'm merely expressing my view that he's not. When some people got mad that I said so, I only then pushed back on their view and said they've watered down the meaning.
How do you arrive at the subjective that one is generational (pre-draft)? I think it's one of those things where you know it when you see it. Sounds vague, but it should be obvious that a player has other-worldly talent in one area that hasn't been matched in the sport in a long time or never. Ovechkin's shot or McDavid's speed are two examples. With Bedard, the shot is his best asset, but is it like Ovechkin-caliber? I'd say no. And there are obvious limiting factors that effect his case. He's 5'10 (rounded up) with adequate skating/compete for his size. If you go the Crosby route of generational without the otherworldly asset in any one area, is Bedard really at that level of complete in every area of the game? Not sure he is. There are some minor limitations to how he plays. So in total, I think you get another player below the generational threshold, yet one that very reasonably could have a HOF career.
Now with the course of a hockey career, I think it's very fair to arrive at the conclusion that you can be generational based on stats and accolades. Thats because we are comparing players in the same arena. They all play in the NHL. They all compete for relatively the same awards. However, a generational career (impossible to discuss for a player who hasn't played an NHL game) and a generational prospect (relevant to a discussion of a just-drafted 1OA) are different discussions.