The Forgotten Realms does seem to operate on the notion that Adventurer is a well-established and well-understood profession. It's a pretty weird place, and based on the novels I've read, I wouldn't be at all surprised if someone had actually performed such tests.
More generally, though, the basic premise of a class-and-level system is that you're doing all sorts of things all the time. A level is a long enough period that you should have used all of your skills and spells along the way, before gaining the next level. (You don't get better at casting spells by killing goblins; you get better at casting spells by casting spells, which you are assumed to do in proportion to how many goblins you kill.) To that end, a wizard who is almost to level 5 will be more knowledgeable and more competent than one barely past the level 4 threshold; it's just that the game mechanics are insufficient to reflect that difference.
To the wizards who actually live in that world, it seems reasonable that one might be able to figure out that they can almost cast Fireball. Narratively speaking, they are almost competent enough - in terms of magical energy, or skill - to prepare it. They're certainly better than they were two weeks ago, when they finally mastered Flaming Sphere. They're aware of the in-game reality which corresponds to Experience, because it's literally just experience.
