they don't need official records, but you need to be able to defend whatever you put on your app. so if you say you work 4 hrs/week at the local hospital, then in theory, they should be able to call your hospital and find that you've worked approximately that much time there.
How often it's actually checked, i dunno. i would imagine not that often. its usually what you get out of the experience that matters more than simply hours. If you work there enough,i don't think anyone will doubt your commitment to the place you're volunteering/being involved at.