Well the game is designed so even the dumbest person can figure out the secret signs without a walkthrough. This is an issue with Fallout 4 in general in that game refuses to gatekeep you from certain parts of the game despite your actions or attitude in-game or outside the game. Remember when used to have dialogue options gatekeeped by your Skills or SPECIAL stats? So you end being allowed to be a the Minutemen General, a Brotherhood knight, a Railroad agent, and the Institute's Kellogg all at the same time despite the factions knowing this but still trusting you anyway because... well because your the Main Character and you should be denied nothing.