To compare the photo before and after the postproduction

While in the edit mode if you press the "m" key it will show the unedited version as long as you hold down that key. Toggle between the edited and unedited versions with the "m" key.

Like this demo of the Retouch tool:

The m's don't show when you're toggling, only in the screen capture.

It is not easy.

First you will have to duplicate the image so you can view both images*. Then, after your editing, you need to export both images and open each one in Preview so you can view them side by side.

*If you are using the image straight from the import as the comparison image you can skip this step and use file->export->export unmodified originals

It is a pity that Photos is only offering the toggle between the edited version and the original as a way to compare the original and the edited versions. It is making it hard to compare the colors and the composition of the photos, because it is putting a strain on the short term memory. And it is impossible to compare two similar photos this way.

I am using the "duplicate" method described by Keith. This is particularly useful, when I want to add further adjustments to an already edited photo. Then I have to compare two edited versions and the "M" does not help. An added bonus of the duplicate is, that it will save the intermediate version, if the added adjustments are no good and I want to revert them. Frequently I do not export the photos to compare them with Preview as Keith recommends, but I simply select both versions and hit the "P" key to open a print dialog. Then I can see both versions arranged side-by-side on the print layout and can compare the colours and the composition.