How can Mail.app in Mavericks show a messages plain text alternative?
Apparently there is no way to do this with mavericks' Mail.app.

The option in the menu is gone and setting it by hard in the plist file doesn't seem to work anymore either, according to this post and this post.

Unfortunately you have to switch to a different mail client if you want to have proper multi-format support.

Or, for testing purposes, you could just send you the email in plain text and see how it looks and then verify that both parts are present by looking at the raw mail text (which Mail.app still lets you do).