An in-depth documentation of Discord's Developer Mode
If you're going to make use of the Discord API, for instance to create a bot, then Discord's Developer Mode will be of great help to you.
Enabling Developer Mode
Enabling Developer Mode is easy. Open your Discord settings (the next to your name at the bottom left) and click on Appearance. There you will find Developer Mode. Click the toggle to enable it.
On Android, you can enable Developer Mode by going to your Discord settings (the next to your name at the bottom left) and click on Behavior. There you will find Developer Mode. Click the toggle to enable it.
Developer Mode adds a simple Copy ID option to your context menu. Right click on any server, user, message or channel to get its unique ID.
NOTE: Developer Mode is not available on iOS at this time.