As mentioned in the comments, one common way to demonstrate a virtual and real crossover capacity is barcodes. However that's not entirely without cost because you'll have to redo all the posters and wall art to include the barcodes, and they'll steal from or otherwise diminish the aesthetics of the posters.
Instead, digitised images (photos!) of the posters, with a strong priority given to the first poster they're likely to see on entrance, as buttons/windows to the AR experience, might be the best possible "clean" solution.
Sequence and weight these images of the posters as a well organised array of buttons with a simple scrolling mechanism and you might also wind up encouraging visitors to more deeply explore and discover the extent of the museum.