What I mean is that remembering stuff (like what someone casting a fireball looks like) is what 3rd edition would call a Free Action, like talking. You can do it as much as you like (that can reasonably fit into six seconds) at any time during the turn, and it doesn’t take up any resources (like your reaction). Even though you’re rolling a check, it isn’t using part of your turn.
If it did take a reaction to identify a spell as it’s being cast, that would mean that remembering something would take the same about of time and energy as an opportunity attack.
I had the idea for some Don Quixote style story where your average isekai fanboy gets summoned to another world, and it’s the job of a member of the royal guard to protect this guy. The fanboy is a delusional, socially inept, weak lech who is convinced he’s the main character of an eroge, and the knight (the actual protagonist) has to try and keep this idiot from getting himself killed (like explaining that peeping on the princess while she’s bathing would most likely result in execution, not a “meet-cute”). Hijinks ensue.