From FreeSpace Wiki
Is destroyed delay (Boolean operator) Becomes true <delay> seconds after all specified ships have been destroyed. Returns a boolean value; Takes 2 or more arguments: 1: Time delay in seconds (see above). Rest: Name of ship (or wing) to check status of.
In a directive event, use of this SEXP will automatically prompt the HUD directive to show a count of how many ships still need to be destroyed.