There are 119 historical items in the game. Finding 50 of them is easy enough if you look for shiny objects, open lockers, explore dead ends, and so on. Press
whenever you are close to a possible item to pick it up.
By doing so you the achievement will unlock halfway through the story. You can replay story chapters if by any chance you haven't found 50 by the time the game ends. A reward of 125 GS is quite a lot for such an easy task.
If you'd like to find all historical items in the game, check out the videos in that other solution: