The way you authenticate a piece of software (a font is a piece of software) is you have a receipt of purchase and a license agreement on paper stored on file. Without these 2 options its nearly impossible to verify ownership. In case of digital stuff you still need to have a paper copy of the money transaction and the license key. Then the vendor can authenticate the key itself.
In essence is you have some sort of letter of authentication that guarantees ownership. If you buy stuff from 3rd parties you need to get a receipt and the original receipts to trace the source down. But beware many countries do not allow you to transfer licenses without special permission from the license holder.
In practice its actually pretty hard to validate ownership of licensed items. Best purchase the stuff from the vendor directly WITH the company credit card as this is often a problem if it was bought by somebody else.
PS: I'm not a lawyer, better yet I'm not YOUR lawyer and don't know your jurisdiction so please check with a legal counsel.