Enable the full size window preview in Mission Control

Mission Control groups previews of windows from the same app. By hovering the cursor over a these group and scrolling up, you can see that the group showing more detail in those behind the main window. With the following command you enable to see a full size preview of the contents.

defaults write com.apple.dock expose-cluster-scale -float 1

To make the command take effect, you need to restart the Dock.

killall Dock

Revert to the default setting:

defaults delete com.apple.dock expose-cluster-scale

Restart the Dock