There's really no way to see who's on a server other than actually joining the server.
But you can see who plays on some of the servers by going to the Global Player Rankings website.
Not all of the servers are listed but it does show a lot of them.
The Global Player Rankings combines the player statistics into a single database, allowing players from different online game servers to compete against each other.
It's up to the server admin to install FLstat v1.5 and then submit the information for inclusion.

What FLstat does is show;
Top 1000 Killers - Top Players based on their total number of NPC kills.
Hall Of Fame - Top Players who haven't been online for 6 months.
Weekly Kills - Top Players based on kills made this week.
Top Clans - Top Clans based on combined average number of NPC kills.
Search the full online database for any Player or Clan.
List of Servers, showing which mods they use, and their online status.
Most Active Servers ranked by how many players have been online this week.
Server Info - official title, current players online and Web Launcher link.
Click on any Player, Clan or Server name to see detailed statistics.
Data is updated every 6 hours (individual sites may not be updated as often).