On specific Quick Keys, there is a button combination you can perform which includes the dial button, set switch button, and the power switch. 

While holding down the dial button and set switch button, slide the power switch, and this will bring up a menu that you can toggle through with the set switch button. On the last screen will be the serial number of your device.  This will only appear on certain Quick Keys, unfortunately.

With the Quick Keys turned on: 

You will need to do the following at the same time;

Press and hold [1] the Dial button, [2] the Set Button, and [3] slide and hold the Power On/Off slider in the off position, and you will see a new text to the right. 

Once you see this text or text that looks similar, press the [2] Set button until the serial number is displayed. 

[4] The serial number will look like this.

Some early models of the Quick Keys will have a blank serial number screen or one that only lists SN: like the image to the right. 

If you have the box you can find it on the bottom of the box as shwon i nthe image to the right. 

If you have any additional questions, please email Xencelabs Customer Care at CustomerCare@Xencelabs.com