Check the sticker on the bottom of the laptop
Check the sticker inside the battery compartment if the battery is removable
Press Windows + R, type cmd, and run `wmic bios get serialnumber`
Open Command Prompt and run `wmic csproduct get identifyingnumber`
Open HP Support Assistant and look under device information
Enter the BIOS/UEFI by restarting and pressing Esc or F10, then find the serial number in system information
Check the original box or purchase receipt
Check the HP warranty or support page using the product details