Do you know the whole game by heart?)
Pretty much. Doing support for Faerin on Discord and on Steam, you kinda have to know the material frontward and backwards in order to help out the players.
The only thing that I don't have down to an art yet are the stats required for certain things (except for the bonus ending). So if a hint isn't showing or something is bugged in the game, looking at a save helps there.
For the more complicated stuff, when I look at a save, I compare mine to the one I'm working on to see what might be missing (pics, flags, etc..). I have most of the images in the game committed to memory, but for others I'll refer to my save often.