What are you trying to accomplish and from where are you monitoring it?
If you mean from the workstation in question, you would have to run an application that monitors the power state and send a notification when it sees the appropriate change. Take a look at these, they might help:
Detect Windows Going in to Hibernate/Sleep
MSDN Power Broadcast

Ping only gives you a positive. A negative doesn't necessarily mean the workstation is down. The ping could be failing somewhere in between the originating and end-point workstations.

If it is important, I would have an application on each server you want to watch that monitors the power state and periodically report its status to a central reporting server. I think that is about the only reliable method.