How do I see what devices are signed into my account?

Active Sessions lives at Account Settings → Security. Scroll past the password and 2FA sections to find it.

Two things to know:

• Only one session is live at a time. When you sign in on a new device, the previous session is automatically terminated. So the "active" list usually has exactly one row (your current device).
• Past sessions are kept as a history. Click Show Inactive Sessions to see what devices, browsers, and locations have signed in before.

Use the history to spot anything you don't recognize. Locations can be fuzzy (VPNs, mobile networks), but a totally unfamiliar device is worth investigating. If something looks off, change your password and enable 2FA immediately.