How to you know if someone is online on Discord?

Discord has 5 status icons that are pretty simple representations of user status.

They are as follows: Green = Online. Yellow = Idle. Red = Do Not Disturb. Purple = Streaming.

You can modify your status by clicking on your portrait in the lower left corner of the Discord screen. A menu will pop up letting you choose which status you want to be seen as.

A red icon with a line indicates that you don't want to be disturbed. Keep in mind that you only get put into idle status when you've been away from your device for a while- similar to being moved to an AFK (away from keyboard) voice channel. And while Offline doesn't have a dot, you'll see the user's name and avatar grayed out pretty thoroughly.