Do yourself a favor by enabling Activation Lock for your iPhone. If you aren't aware of the process, then it can be done by enabling “Find My iPhone” option in settings. The basic job of Activation Lock is to protect your device from any kind of misuse when it is stolen or lost.
By having an Activation Lock, you can be assured that no one will be able to use your device. In addition, they won't be able to even erase the device and use it as new. Now you know how important it is. However, if you aren't sure whether Activation Lock on your iPhone or iPad is ON, then you can follow the guide below to check the status.
Update: Apple has removed Activationlock page from iCloud.
How to Check iPhone Activation Lock Status
Step #1. To check the status of Activation Lock, open web browser on your PC or Mac and visit https://www.icloud.com/activationlock/.
Step #2. After the page is loaded, you'll need to enter either IMEI number or the serial number of your iPhone or iPad. You can find IMEI/Serial number by tapping to Settings → General → About.
Step #3. Once you enter the IMEI number or a Serial number, the next step is to enter the verification code. It's a random set of numbers or alphabet, just to prove you are not a robot. So enter it and click “Continue.”
That's all; if the Activation Lock is enabled, it will display a message “Activation Lock.” You can check other device's status by clicking the link that says “Check Another Device” and repeat the process.