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Type "/seed". So long as you are an administrator on the server, that should do it. I am not sure if "cheats" need to be enabled in single player for this to work, but I'd assume they would.
They sadly need to have cheats for that command. The way to get it without opening a LAN or command is to click on "Recreate world" then look at "More World Options." The seed will be there. You do not have to save the re-created world.