When you are working on a multi-layered document, it's sometimes difficult to see the layer contents in the thumbnail image. Holding the Alt key (Mac: Opt key) on your keyboard while you click the Layer Visibility icon (looks like an eyeball on the left side of the layer) hides all of the layers except the one you clicked on. This allows you to see exactly what is on that layer.
Alt click (Mac: Opt click) the Layer Visibility icon again to see all of the layers again.
And here's an even better trick. Hold down Alt key (Mac: Opt key) while clicking on the Layer Visibility icon of one layer. Then press the right or left bracket key on your keyboard without letting up on the Alt (Opt) key. Pressing the right bracket key will show you one layer up the Layers panel, while the other layers will be hidden. Pressing the left bracket key will show you one layer down the Layers panel. You can scroll up and down the Layers panel, seeing what’s on each layer this way.
These tricks can help you identify the contents of each layer even if you can't see the thumbnail clearly.