Impossible. At best, you can take a guess. But you can guess very reliably because HTML must have an HTML element.

If you don't know if it's text or HTML, then you're surely dealing with bytes, so you need to handle UTF-16le, UTF-16be, UCS-2le, UCS-2be, UCS-4le, UCS-4be: 

/<HTML|<\0H\0T\0M\0L|<\0\0\0H\0\0\0T\0\0\0M\0\0\0L/

[download]

If you're somehow dealing with decoded text: 

/<HTML/

[download]

Update: No, that's still not good enough. A text version of this very post would fail, for example.

In reply to Re: Distinguish between HTML and Plain text by ikegami
in thread Distinguish between HTML and Plain text by vit

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
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