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.