<p>I want to run text through a filter to ensure it is all UTF-8 encoded.  What is the recommended way to do this with PHP?</p>

Your question is unclear, are you trying to encode something?  If so
utf8_encode
is your friend.  Are you trying to determine if it doesn't need to be encoded?  If so,
utf8_encode
is still your friend, because you can check that the result is the same as the input!