How do I check for OEM (pre-installed) licences?
Check every single PC for a Certificate of Authenticity (COA) label as proof that it has a legal licence. You may find these stuck on the chassis of the PC, the original CDs or the documentation that accompanied the CDs. PCs more than three years old probably won’t have COA labels on their chassis: they will be on the cover of the manual. If the software is four or five years old, they will be on a separate COA document.