There are 2 ways to check the serial number of Melody Tag.
Look at the cover of Melody Tag, it will show just beside the QR code. An example of the serial number based on the picture below is 53:F0:AB:83:52:00:99.
Download NFC Tools App (with the orange icon) on your mobile phone. Open the App and under the READ section, put your Melody Tag close to your phone. Click on the Serial number column and select "Copy to clipboard".