I hope that you have only one version installed on your backup server ;-)

If you mean the client versions, this is pretty simple:

1) GUI

    From  NMC > NW Admin > Configuration tab > Clients

    Right-click the client list's title raw and select the columns Name & NetWorker version

    Sort by the column NetWorker version

2) CLI - like in this example:

D:\>nsradmin > D:\clients.txt

. type: nsr client

show name; networker version

p

q

D:\>

     Then process the text file appropriately.

You can also use nsrpush and just run inventory.