Ask for recommendations.
Ask lots of questions of the agent.
Look for a 'Cruise Specialist' so you know they've been on alot of cruises and can help with all cruise lines, ships, and ports of call.
It's like trying to find a good auto mechanic, accountant, or dentist. Search for a good reputable one you feel comfortable with and can trust.
47 Cruises & Counting! Favorites: Paul Gauguin to Tahiti: Uniworld River Cruises in Europe; any of the Celebrity Solstice-class ships; Holland America for 12-nights in the Baltics & Russia; RCCL for 14-night Greek Isles, Turkey, & Croatia; Holland America for 14-day Alaska cruisetour; 10-night Canada/New England cruise; 21 days in Hawaii including a 7-night NCL cruise; Oceania for 25 days in Asia; & 3 months touring Europe by train. And many days spent in all-inclusive resorts!