ISON

Usage: ISON <nick> [<nicks>]
Will check to see if one or more of the nicknames are currently logged into the chat network. It will report back all of the specified nicks that are online.
Examples:
/ison Surel
/ison malakian scott-s kAyleigh
This is fast way to check if a nick is in use. You could use the WHOIS command, but then you would also get multiple lines of information you might not care about, and could only check for one nick at a time.