When you create a campaign with Campaign Monitor, you can select to send it in plain text, just html or multipart text+html formats. Now testing an email that is just text is pretty straightforward, and testing the html portion is also easy, but how do you see the plain text part of a multipart email? Here's how to check what people will see if their email client is setup to show text only.
Testing using Thunderbird
Thunderbird is a cross platform, free email client, and it lets you easily view the plain text version of a multipart email.
View / Message Body As / Plain Text
Testing using Apple Mail
Apple Mail has the same functionality built in:
View / Message / Plain text alternative
In both clients, you can swap back and forth between html and text views.
Outlook: Unfortunately, there does not appear to be a way to view the plain text in Outlook. We'd suggest you just grab Thunderbird and use it to check your campaigns. Now that Campaign Monitor remembers your test addresses it's even easier to set up a consistent test process.