Let me start with this picture:
This is actually the original RAW image without any editing
I open the picture do my editing. During the editing I get to see the before and after by clicking on this icon. When it's up, it's the before situation. When it's down, it's the after situation.
With all my editing done, I hit the backslash key (\) to see the before and after image. Click it once and you see the before situation. Hit it twice and you get to see the after situation. For Mac users it's Shift + Alt + 7.
Mind you: you can also see the before and after by clicking on the icon underneath your image:
The fastest way to compare the before and after is actually pressing the 'Y' on your keyboard. Click it twice to see just the after.