Whether you’re in an open office where colleagues regularly wander past or live somewhere—like a college dorm—where you may feel comfortable leaving your laptop unattended in the presence of relative strangers, it can be all too easy for someone else to sneak a look at your computer.

If you want to keep your device secure in communal environments, your best bet is to understand how to stop unauthorized access in the first place. Still, there’s some detective work you can do if you suspect someone else has been using your device.

Since prevention is better than a cure, you ideally want to prevent others from accessing your laptop in the first place. A simple way to do that is to lock your laptop behind a password whenever you step away from it.