You may find the model name & seven-digit serial number on the original packaging of your product.
Alternatively, you can also find the model name & serial number on the product or on the case. Below are some examples of where you can find the model and serial number for different types of headphones.
You can usually find the model name & seven-digit serial number either on the headband or inside the head-padding, as shown in the image below (example of WH-1000XM5)
Truly Wireless Headphones
The model name & seven-digit serial number are located either on the bottom or within or behind the charging case, as illustrated below.