find out license of an old notbook

[chhal]

The OS was competently wiped up.

How can I find my serial?

[BastianW]

It depends on the model. Normally there is a sticker on be device additional the license can be added by the manufacturer in the BIOS. On top of that Microsoft often save the license which is bound to a hardware in there online database. So if you install Windows 10 for example and has an working internet connection the info's can be fetched automatically from that database during the windows 10 installation.

[WAWood]

@chhal 

As @BastianW already indicated, it depends on the OS version.  With Win10, a reinstall will reactivate after contacting the MS activation servers when you are online.

With Win8x, the license was embedded into the firmware of the motherboard, which you could then retrieve using the tool downloaded from this link, but you have to boot into an OS to do that:  https://www.tenforums.com/software-apps/2577-showkeyplus.html

With Win7, unless you can read the sticker, you are out of luck as that was stored in the Registry, which was removed when you lost the OS.