This will add subscribers from the response to the "subscribers" hash table. Before you send the command, you should clear the table so that users no longer subscribing are removed. Afterward you can see if somebody is a subscriber with $hget(subscribers,$nick)

Code:
alias subscribers.update {
  hfree -w subscribers
  raw TWITCHCLIENT 2
}

on *:text:SPECIALUSER & subscriber:?:{
  if ($nick != jtv) return
  hadd -m subscribers $2 1
}