Open the world you want the seed for
Type `/seed` in chat if cheats or commands are enabled
Press Enter
Read the seed shown in chat
In Java Edition, open the world to LAN and enable cheats if needed
In Bedrock Edition, pause the game and check the world settings for the seed
On servers, ask the server owner or use a plugin if allowed
If commands are disabled, enable cheats in a copy of the world and use `/seed`