Ah, you misunderstood.
If you enclose say <p>some text.</p> inside like <? <p>some text.</p> ?> then mod_php treats it as a comment and never sends <p>some text.</p> at all. But if it fails you see some text.?> displayed by your web browser or <? <p>some text.</p> ?> if you look at it raw.
The point being a simple test for php being present. And one I had tested on my own box.