You can find out here by entering your device's ESN, MEID or IMEI/MEID number.
To find the IMEI dial
To find the IMEI number, dial *#06# on your device's keypad. Then, enter the 15-digit number in the area provided on this page.
You can also find the ESN, IMEI or MEID numbers on the box the device came in, on a sticker behind the device's battery, or in the About section of your device's General settings.