Look at the back or the bottom of the product box. There will be a white sticker containing barcodes and regulatory information.
Your JBL Tune 110BT has an inline remote control (with volume +/-, play/pause, and power button). Sometimes, the serial number is placed here instead of the neckband. How to Check Serial Number on JBL Tune 110BT
The label may be gray or black with light etching, making it hard to read. Use a flashlight at an angle to see the text. Look at the back or the bottom of the product box