How can I detect a text/plain base64 email message with no other text parts?

[Loren Wilton]

I get a lot of spams, and a major characterisitc is they only have 
text/plain that is base-64 encoded.
Since I live in an area where base-64 encoding is basically never necessary, 
almost all base-64 encoded text parts are major spam signs.

Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: base64
Thanks,

       Loren