Microsoft Volume Licensing agreements contain specific clauses that allow Microsoft to give notice ( 90 days I think ) that they are going to come on site and perform a software audit. You don't have to sign these agreements, but then you lose a whole load of benefits ( pricing, downgrade rights, etc )
We currently have a customer that has been put on notice by Microsoft that they are going to be subject to a random audit. We have 4 weeks to supply a full list of software in use, list of purchased licenses and other evidenced documentation that they are in compliance. If MS aren't satisfied, they can send a team in to do an audit.
This type of thing applies to all sorts of domains. IT, finance, insurance. If you look, you'll find any large business has signed countless contracts that allow for on-site inspections to ensure compliance.