Where can I find the part number or serial number on my earbuds?

First, for wired earbuds, take a look on volume control segment of the cable. Normally, there is a sticker or etching somewhere there, which contains a part number and usually the product name next to it. 4Pxxxx is the most common structure but it may start with something else. Significantly older models may instead start with "KHX-" or "HX-" alongside a number starting with "3500xx".

If you see an indent that looks like it was supposed to hold a sticker, but do not see the sticker, the sticker may have fallen off.

For wireless earbuds, the first place to look is actually the charging case. Open the lid, and you may see a sticker / etching of the product info either next to where the earbuds slot in, or on the underside of the lid. If you don't see it there, check all around the outside of the case as well. You're looking for the same kind of info as what would have been on the cable sticker above. As before, if you see a small indent that looks like it might have held the sticker but you don't see one, the sticker may have fallen off.

Wireless buds may also have the info sticker on them, or etched into the buds, directly. If you still haven't found the info for your wireless buds, take a close look at each earbud in a well-lit area.