Good question! The mde app used to tell you the wait time and FP return times and you could easily see when they were gone. But that was when the FP was dealt out in order. Now there is so much shuffling and changing on the spot that I would dare say they are never actually 'out' when one could be cancelled 10 minutes from now.
That said, the attraction will grey out if a FP is not available at the moment.