How to find your world seed
The seed is the ten-character code the world was generated from. Same seed, same world, so once you have it you can open the whole map in the seed map and see every altar, trader and dungeon before you have sailed anywhere.
Start the game, pick your character and open the world list. Each world shows its seed under the name. That list is the one place the game prints it without any tricks, and it is also where you type a seed of your own when you create a new world.
You do not need admin rights. Join the server and press F2. The overlay that appears lists the world name and its seed alongside the frame rate and ping. Anyone in the world can read it, so this is the answer for a friend's server or a public one.
The console has no command that prints the seed, and the seed is not in any log you can reach as a player, so F2 is the whole trick.
A world is two files, name.fwl and name.db. The small .fwl one holds the name and the seed. On Windows they live in %USERPROFILE%\AppData\LocalLow\IronGate\Valheim\worlds_local, on Linux in ~/.config/unity3d/IronGate/Valheim/worlds_local, and a dedicated server keeps them next to its start script. Drop the file below to read the seed without opening the game.
Paste it into the seed map for the full world, or run it through the seed finder to compare it with a hundred thousand others. A seed is at most ten characters: anything longer is cut by the game itself, so a longer string is not a world that can exist.