Distinguishing Barcode Scanners from the Keyboard in WinForms
Check out this thread. There's everything you need there.
If you'll be using this particular scanner, you can setup it to send a specific signal at the end of the input. To setup your scanner gun you need to read the documentation for your device, probably provided on the website of the manifacturer.
Another solution if you will be using the same scanner gun.
If your clients are going to use a lot of different scanners, it would be slight difficult to set everyone of them and track their input. In such case, you have to do it with counting the time between the keypresses (Windows cannot distinguish between barcode scanner and normal keyboard). But you still have to know the suffix of the input the scanner sends.