When you’re cleaning up a mask, it’s often difficult to see if you’ve missed any pixels. Here’s a way to make them easy to spot. With your layer mask selected in the Layers panel, click the ƒx icon at the bottom of the panel and choose Stroke. In the Layer Style dialog, select a bright color like red. Set the Size quite large to 50 px or so and click OK. Now every single masked pixel will have an outline around it and you can easily see how nice (or not) your edges are as you paint them away with your masking brush. 

This tip originally published in Colin Smith’s “Photoshop Tips” column in the March, 2023 issue oPhotoshop User magazine.