Use The Overlay Modes To Check Your Lightroom Masks
Lightroom masking is powerful and helps photographers emphasize subjects and craft stronger images. The mask overlay defaults to a red color showing an area the is mask. Depending on your photo, that red overlay may be difficult to see. There are three other color overlays for masks in Lightroom - green, white, and black. Cycle through these using the keyboard shortcut Shift-O to double check the precision of your mask.
See this technique in action in the short video with this article.