One way of effectively demonstrating the impact that your research has had on your stakeholders is to collect testimonial evidence. This generally takes the form of a letter from a collaborator on headed paper, although e-mails are also acceptable.
It can sometimes feel awkward to ask collaborators to write corroborating statements of this kind and this is why researchers often leave it to the last minute to request this information. Don’t make this mistake: if you leave it too late you may find that the main contact for your research has left the institution, may have retired or even passed away. You should therefore capture all evidence from your stakeholders as soon as you can.