Is there a report that shows all users that have call forwarding enabled?

Don't think that's possible. My best suggestion is to look for a report showing users with 0 inbound calls. People with 0 inbound calls are either not being called or they are forwarding their calls elsewhere, or they are on DND (Do Not Disturb).

You could further narrow it down buy applying an additional filter - 0 missed calls (this would exclude people who are permanently on DND and all of their calls are getting missed.