But if you take the CPU out of the system and clean teh TIM off you will be able to read the SL number there.
Try this utility to see if it identifies the CPU.
Note: Some cpu may be a Tray/OEM version, it may not recognise it.
Otherwise, like CptDogfruit stated - Read the markings on the CPU. Thanks Cpt for helping out.
Check the SL number on:
All the best,