HOW-TO Check if user is subscribed to thread?

[addamroy]

I'm looking for some kind of conditional in SHOWTHREAD to check if a user is subscribed to the thread.

Basically as of now I have a 'subscribe to thread' button that shows only to members. I was hoping there is a way to add one more conditional to check if the user already subscribed to the thread, in which case the button would disappear.