Hi deamonblade,
If you are working for a company, you will usually have timesheets, basically some way of logging cost/expenditure against a project.
You can then compare this against the previously purchased licence.
If you are freelancing, it depends on many things, in Australia you need a "business number" and need to log your hours and any costs against that project ( the product licence for instance) as this effects tax.
If you have no record this will be hard, you could try and prove that your file windows time-stamps ( date modified ) were within the licence window, some programs also include "meta" tag information with time-stamps.......
You will need to be more specific for a more detailed answer.
Regards,