Switch the layer visibility off and on again in the Layers panel with the eye icon. This helps you see if there is more of the image you need to work on.
Create a new, separate layer between your two existing layers. You can do this by pressing the plus sign (+) icon in the Layers panel.
Fill the middle layer with a strong color that does not appear in the photos you are working with. This will show any parts of your top layer that are not erased properly.