In the Scheduling Tool, the only way you can tell if a resource is scheduled for time off is this light grey bar that stretches the width of the time off dates. I've noticed that the grey bar is buried in within other tasks that may be assigned, so it's not easy to tell if a resource is available or not. I suggest making it more noticeable and moving the "time off" bar to the top of the resource's list of "tasks."
I'm happy to submit a screenshot of this if needed.