Answer: An Opacity Tester!
In printing, opacity refers to the degree to which a sheet prevents dark print areas from “showing through” to the other side of the paper under normal lighting conditions (aka “show-through”). This is especially important if one side of your piece has a lot of white space.
A simple test to check opacity – especially when comparing two sheets side by side – is to place an opacity tester (a printed sheet with a rich black pattern – usually the one above) under the papers. You will easily see which sheet has the greatest opacity and least show-through.