The bleed box, as all the other page boundary boxes in PDF (crop, trim, etc) is a rectangle defined by 4 corners coordinates. I suppose the bleed size you want to know is the difference between the trim and bleed boxes? Because these are rectangles, it may result in 4 different values, i.e. left, top, right and bottom bleed sizes. Do you want all the 4, just the maximum, or this is just a scenario not happening with your PDFs?