Determine if an action has completed after being queued in a Fiber Pool? [.NET4]

I am building an authoritative server using Photon Server SDK. Using a fiber pool I can queue actions to prevent race conditions and untimely access. However, the PoolFiber class does not implement a method to determine whether or not the action passed into the queue is completed. So is it even possible to determine whether or not an action has executed?

Note: I tried spin waiting, but I have read that it is very inefficient. So perhaps there is an alternative?