This is HTML:

Please use <code>...</code> tags around your code.
[download]

This is text:

Please use <code>use strict;</code> in your code.
[download]

How can one possibly correctly identify them programatically?

PS - This is the reason Atom is better than RSS. RSS doesn't provide a mean of specifying the content type, so it can't distinguish between text and HTML content. Clients have to guess. You could take a peek at how RSS clients do it, but I suspect they might work with less ambiguous content than you.