If song name is $!null, then consider it as winamp isn't running...
Code:
on *:dialog:name:sclick:button-id:{
  if ($dll(swamp.dll,WinAmpGet,TRACKFILENAME) != $!null) {
    topic #chan Winamp now playing - $nopath($ifmatch)
  }
  else {
    !.echo -q $input(Nothing is being played,o)
  }
}