What is my current service pack?

We have a NW5.0 server with 5 user licenses. We also have an additional 5 licenses to add-on, however, all attempts to load these have failed. We think that perhaps reloading the current service pack, or the next one may help the situation. However, since neither of our IT folks have NW5 training, haven’t been able to determine what level service pack is currently running. Any suggestions?