You may never even think to steal a phone, but who’s to say that you wouldn’t purchase a stolen phone by accident? Here to help you ensure that you don’t break the law, inadvertently or not, is a new tool from the CTIA (the nonprofit representing the U.S. wireless communications industry) that lets you check if the phone you’re interested in purchasing came from an illegitimate source. Aptly named the Stolen Phone Checker, this tool quite simply allows users to look up if a device has been reported as lost or stolen.

The straightforward site is free and easy to use — simply input your device’s IMEI, MEID, or ESN. Each of these codes are unique to a mobile device, and every mobile device must have one of these ID umbers. Finding them might be a bit tricky, though. For example, if you have an iPhone, the number may be printed on the back or your device, but some other phones may require you to dig through the settings menu to find the code.

In any case, once you’ve located the specified string, just head over to the Stolen Phone Checker, input those digits, check the “I’m not a robot” CAPTCHA, and hit submit. You’ll then be told if your phone is safe to use, or has been reported stolen.

One caveat, however: if a stolen or lost phone’s original owner has not reported the device missing, the tool won’t know about it. That said, Stolen Phone Checker will still likely be useful the rest of the time.

So why is it so important to know whether or not your device comes from a legitimate source? Quite simply, if you try to activate a stolen phone, it just won’t work. In fact, American phone carriers have a collective database of stolen phone IDs, and if your device happens to match one of these numbers, it won’t be able to join a wireless service provider’s network. That means it’s key to check the authenticity of that phone on eBay before you buy it, and you can do that for free (up to five times a day) with Stolen Phone Checker.