Ison
From Scriptwiki
Checks if a certain nick is online / in use.
/ison <nick>
The result is returned through Raw 303 where $1 will be your nick and $2 will be the target nick if it exist, or nothing. See working example below.
Example
The following example will modify the returned output of an /ison request to a more user-friendly format.
raw 303:*:{ if ($2) { echo -a $2 is online. } else { echo -a The user is not online. } haltdef }