Measure the Drop
Lay a long level "downhill" on the floor, with one end of the level on the level part of the floor and the other end at the lowest point. Raise the lower end of the level until the bubble is centered, then measure the distance from the bottom of the level to the floor to find the depth of the dip.
If the level isn't long enough, lay a flat board along the floor with a level on top. Raise the board until the level reads true, then measure from the dip in the floor to the bottom of the board.