The /cgamemode command lets you check the gamemode of other players on the server and list all players currently in a specified gamemode.

usage tree:
/cgamemode
├─ query
│  └─ <player>
└─ list
   └─ <gameMode>

• Check the gamemode of another player: /cgamemode query <player>
• List all players with a specific gamemode: /cgamemode list <gamemode>