I recently volunteered to do a particular task for a charity. I made the first phone call and established a relationship with a potential donor. Don't do that. If you doubt that a volunteer is doing the job properly, follow up with that volunteer, not with an outside party. On the other hand, some volunteers are very delinquent when it comes to telling anyone what they are doing.
Some volunteers even refuse pointblank to give progress reports once they have been assigned a task.
Never become bureaucratic First of all, do not become bureaucratic and stop using a very good volunteer simply because they aren't good reporters. Secondly, whether you are a paid volunteer leader or a volunteer yourself , always commit any assignments to writing. Such written confirmations need not be legalistic. I hate asking volunteers to take their time to provide written reports on what they are doing, so if a written report isn't convenient by date XX, when I have to make a report to the Chairman, please give me a call by XXX to tell me how things are going. Volunteers are not all built the same way.