Quick Mask is another great tool that lets you see what your layer mask is doing relative to your image.
Start with your open image and make a selection somewhere. Anytime you have the "marching ants" of a selection, you can use Quick Mask. You'll find the toggle right underneath the foreground/bakgnd color swatches in the main tool bar.
When you activate the Quick Mask you'll get a pinkish hue covering all but the area within your selection. -Masking the non-selected areas. You can now grab a brush and paint white to erase the mask, black to fill-in additional mask, and the shades of grey to partially mask pixels.
If you have a layer mask already, ctrl+click the mask thumbnail to load it as a selection. You can then activate the Quick Mask to see what is actually getting masked. This is editable with a bk or white brush too.
Sometimes the selection is very complex. Quick Mask makes it easy to see what's actually going on.
If you are working on an image with a lot of reds and pinks, then the default pink color of the quick mask makes it pretty much useless! If you double-click the QM icon you'll open the QM preferences box. Double click the color swatch to change the QM color.