We have a very old home with knob and tube wiring. There is a main (hot) wire and a white wire but no grounding wire, however, there is a grounding wire coming off the fan motor. How can we ground this?

You can't, unless you run new cable with aground wire that gets grounded at the source. It is not imperative that the fan be grounded, it will still work fine.

Issue is if it ever shorts out, no fuse or breaker will trip, the metal housing will become energized. The chance of this occurring is nil. In the future, if you need to work on the fan, just be sure the circuit is off.

I would be more concerned about the ungrounded outlets you may have thruout the home. Do you have 2 wire or 3 wire outlets? If 3 wire, are they actually grounded?

I don't ground fans in these situations either, but don't tell anybody!

We have a very old home with knob and tube wiring. There is a main (hot) wire and a white wire but no grounding wire, however, there is a grounding wire coming off of the fan motor. How can we ground this?
Very simple screw the green into the box that the wires come out of because your pips are grounded that will do just fine for now, but I would think about getting new wire soon