Another good source is onboard the ship. You have cruisers at your table every night and all of them have booked at least one cruise, the one they're on. Many will often be seasoned travelers with insight into more than a good agent, they often can tell you a lot of the ins and outs of the ports, excursions, shopping, etc.
I've learned a lot from fellow travelers over the years right at my dinner table. If they have a great agent, they'll be proud to recommend them. If not, they'll also share that information.