I don't think it is always trivially easy. Sometimes the narrative is explicitly that the character encounter something completely unknown to them, like my 'orc' example from the earlier. Granted, if one wants to not fuss about the IC/OOC knowledge gap in such a situation then the GM can change the monsters. But not everyone always play that way. I have played in many Cthulhu games where the characters encounter eldritch lovecraftian horrors and are utterly flabbergasted by them whilst the players are Cthulhu aficionados and fully well know what they are.
Why does the narrative have to be so explicit though? Why can't the players respond to the Lovecraftian horrors with what they know as players? Does it change anything about the running for your life and going mad? And even then, could it not be that one of the characters read about Cthulhu? In the fiction of Lovecraft's work, there are books referring to Great Cthulhu. It is not impossible or uncommon for player characters to have briefly gazed upon such esoteric texts. Such knowledge is as rare as you want it to be. In several of Lovecraft's stories, protagonists know of the necronomicon and have read it.