For security reasons, Apple Devices uses iCloud service, this cloud service gives many features, one of those features is “Find My iPhone” service , if you enable this service you can find your device if lost or stolen, Also you can access to your iPhone from any where in the world and gives you the opportunity to remove all your files remotely, the best thing is no one can use it.
That’s way before buy old or used apple devices you have to check is not locked. if you buy any apple device with this feature enabled you can not use it, maybe you will lose a lot of money to use it, if not impossible.
In recent days Apple closed iCloud lock check page, and justified it there are who use this service to unlock locked iDevice ,by changing the serial number, Apple is working to solve the problem before the service returns later.
But there is another site you can use it to check if the phone is locked or not, by “IMEI” number.