Paper opacity is the lack of transparency that allows a sheet to conceal print on the opposite side.  weight, brightness, type of fiber and filler in testing can all greatly influence opacity.  Reflectance of paper is measured when backed successfully by a white body and a black body.  The ratio of these two measurements determines the opacity reading.  Typical opacity of 50# white offset is 88-90; 20# business paper is 84-88.  A quick and very basic “home” test is to print something large and black (black and white clip art and or various sized of bolded text) on a sheet of paper, place a blank sheet of the paper to be tested over this printed sheet.  Any print that happens to show through, will show a bit more if actually printed on the currently black paper.  For book printing, take your darkest page and try this test.  Many manuscript designers will change their manuscript elements to decrease the opacity needed for a nice book rather than pay the difference for higher opacity.