I don't think there is a playable demo but there are some watchable demos that are rendered in realtime using the quake engine so it should give you some idea of how it will perform.
Alternatively I think there's a port of winquake which should be an Amiga executable. If you can download the demo data files for Quake then it should work with this executable (like a WAD file for Doom).
You can find all of these on Aminet.