If you have shot against white, and need of a 100 % white background (255,255,255), you can do the following to visually inspect that it is while removing small imperfections. The steps are as follows:
1: Make a levels adjustment layer, with the input values 253, 255.
2: Set layer opacity to 85 %
3: Select dodge tool for highlights with exposure around 4-14 % with brush diameter quite large (600-1600px, depending on image size.) Or just use the eraser tool.
4: Select the background layer and dodge/erase all the stuff that you see is not white...
5: Delete the levels adjustment layer.
Now it should be completely white and clean