You can see a general overview of licensing under Help > About. You should be able to see your license key, the number of seats, the number of server seats specifically, and the number of deployment seats used.
For a more detailed look, you can run a Seat License Status report by going to Tools > Create Report, then selecting the Seat License Status report.
I checked all this but it does not show specifically how many server licenses are in use just seats in general.
I don't believe there's currently a way to look specifically at server seats in use.