Is Office server based or does each end user have his or her own install?
You should be able to open MS Office and find the serial number(s) via the "About" screens.
Then look through any records for "proof of purchase". E.g., the purchase of the server license with some number of included client licenses. Plus any additional client licenses that were purchased.
If no paperwork contact Microsoft.
Also look for any "nag screens" or emails regarding updates and/or subscription payments. I get monthly emails when my payment is due.