Roommates—they eat your food, wear your clothes, and snore really loudly. Sometimes, these opportunists even think it's OK to use your computer when you're not around. If you suspect a housemate is helping themselves to your laptop or desktop, there are some ways to find out. Of course, you probably already have password on your lock screen, but maybe you have reason to believe your roommate has guessed your password or somehow circumvented it in the recent past. Now's the time to start changing it regularly. (And make it stronger while you're at it.)

Meanwhile, here's how to sleuth out trespassing roommates, parents, or pesky little sisters.

Check Your Logs

Almost every personal computer keeps a log—a list of literally every action your computer performs. They're timestamped too, so you can see when each action occurred. The easiest way to catch a suspected unauthorized user is to put your computer to sleep when you leave, the check for "wake" log events when you get back. This mean your computer was woken up, and you'll probably see other activity too. You can study logs to see if any past activity occurred as well. Just think back to time when you're certain you weren't at your computer, like when you were at work, at class, or out of town. If you see logged events for those times, that's a clue pointing to somebody on your machine.

For Mac users, look for the Console app, which you can use to view your logs. When it opens, you'll see "All Messages," and you scroll up to go back in time. For Windows users, open the Event Viewer. Look for the Windows Log menu item on the right. Right click on Systems, and you'll see all activity on your computer.