I would guess that you're talking about a nick completer? Where if you type a nick with a colon after it at the beginning of a line, that nick will be formatted in some way (color, bold, underline, fancy characters around it, etc). If so, there are a variety on the scripting sites (search for nick completer) or someone here may write one for you.
If you mean that it checks every word in a line of text and if the word is a nick currently in the channel, then it formats it in some way (see above for types of formatting), then that's a slightly more involved script, though still not too difficult to write.
If you mean something else entirely, please clarify how your initial input would appear and how you want it to display afterwards.
