iPhones and iPads are expensive items to buy new from official retailers, and it's understandable if you're tempted when friends or even strangers offer you a second-hand model at a bargain price. But is it cheap because it fell off the back of a lorry?

In this article we explain how to find out if an iPhone or iPad is stolen.

The red flags

Is it a suspiciously good deal? We discuss plausible prices for older models of iPhone in our How to sell an iPhone article.

Do you know anything about the person or firm selling the device? Have you or anyone you know dealt with them before? Do research where appropriate and possible - look for positive, numerous reviews or ratings of the seller.

Check if the device is locked or unlocked - regardless of whether it's stolen this is something worth checking when buying second-hand, but if they've been unable to unlock it this may be a red flag.

The factors above should give you a sense of how far it's safe to trust the retailer without further measures. But at the very least you should make sure the device you're buying has been erased and disassociated from a previous Apple ID; thanks to Find My iPhone Activation Lock, which we look at next, doing these things will be impossible for a thief unless they have the password.

Check it's erased and ready for setup

Find My iPhone Activation Lock is a feature that Apple introduced as part of the iOS 7 update back in 2013, as part of the Find My iPhone service.

Activation Lock is automatically turned on when someone turns on Find My iPhone, and cannot be turned off again without the password to the associated Apple ID account. As well as this, it also means the user of the device will need to enter the password in order to erase or re-activate the device.

So when you buy a second-hand iPhone or iPad (or iPod touch or Apple Watch, for that matter, since Activation Lock applies to them too), you should check that this has all been deactivated: that it's been erased, and the link to the old Apple ID account broken. If the seller can't do this, then they don't know the password, which is suspicious; it also means you won't be able to set it up with your own account.

The way you do this is start the setup process as normal. Turn the device on; if it's been erased you should see a screen reading 'Set up your iPhone' or equivalent.

If instead you get the passcode entry screen or Home screen (with the previous owner's app icons), it hasn't been erased - or it's been erased but then they've set it up again, which is not what you want.

So ask the seller to erase it for you. They (or you, with permission) will need to go to Settings > General > Reset > Erase All Content and Settings.

Once it's erased, start the setup process as normal (see How to set up a new iPhone and How to set up a new iPad). Several steps in, you'll be asked to enter an Apple ID, and this is the key: does the last owner's Apple ID appear, or is the field blank?

If the Apple ID keeps appearing, tell the previous owner to turn off Find My iPhone. They don't need to do this from the device itself, by the way: they can do so from icloud.com/find but they'll need to enter their password.

Stolen Phone Checker

The previous measures only work if the previous owner turned on Find My iPhone (which we recommend, by the way). But if you're reading this article from the US, there's another tool you can use to check if the device has been reported stolen.

Stolen Phone Checker is a free online tool that lets you check five devices per day, but is US-only for the time being at least. Enter the IMEI, MEID or ESN for the device in question (here's how to find your iPhone's IMEI number; the same technique works for MEID as well) and it will tell you if it's clean.

We'll update this article if we hear that the service is expanded to include the UK.