Gmail – Show text part of a multipart email in Gmail

An email sent with Content-Type of 'multipart/alternative' (often) contains two copies of the email: one HTML, one plaintext.

Gmail displays the HTML version of the email by default, but how do I view the plaintext contents, just to make sure that it looks right?

I can view the raw text using the "view original" option, but that's not really what I want; there's a lot of escapes visible when doing this, which is not what users would see.