Is there a way to check the accuracy of a layer mask without having to depend just on its thumbnail in the Layers panel?
Yes, there is.
Hold down the Alt key (Option key on a Mac) and click on the mask thumbnail in the Layers panel. Elements will display the grayscale mask in the main image window (instead of your image). Now you can check the accuracy of your masking job -- missed spots, clean selection edges, etc. This is really helpful when you are working on a composite image. It helps you see if there are any problems with the transition areas between the different components of your composite. Ex.: a not-so-smooth transition, an unwanted area included in the transition, etc.
NOTE: To display your image back in the main image window, Alt/Option-Click on the mask thumbnail in the Layers panel.
Here is an example of how this technique works. I applied a Gaussian Blur to the whole image. Then, I created a layer mask and began painting away to hide the blur effect on the sculpture. When I displayed the grayscale mask in the workspace, it was obvious I had missed a few spots.
Elements 7,10,11,12,13,14,15 - Photoshop CC 2017 - Lightroom CC 2015 Windows 7 Pro & Windows 10 Pro