Open the world in Minecraft
Type `/seed` in chat
Press Enter
Read the seed shown in chat
If cheats are disabled, enable cheats or use operator permissions
In Java Edition, open the world to LAN and allow cheats if needed
In Bedrock Edition, use `/seed` if cheats are enabled
Check the world creation screen if you still have the original world settings
Look in the world folder files if you have access to the game files
Use a seed-finding tool if you only have screenshots or map data