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.
Travel Agent/Cruise Specialist w/12 yrs exp and 47 Cruises on 11 cruise lines! Favorites: Paul Gauguin - Tahiti: Uniworld River Cruises - Europe; Celebrity Solstice-class ships; Holland America - 12-nights Baltics & Russia; RCCL - 14-nights Greek Isles, Turkey, & Croatia; Holland America - 14-day Alaskan cruisetour; 10-night Canada/New England cruise; 21 days Hawaii w/7-night NCL cruise; Oceania - 25 days in Asia; more than 3 months touring Europe by train. And many all-inclusive resorts!