Look for a cruise specialist over a general travel agent. It is also worth noting if the agent has a CLIA accreditation such as ACC, MCC, and ECC. These stand for accredited cruise counsellor, master cruise counsellor, and elite cc. These are not easy things to attain.
But then again there are great agents who have not attained the above. My agent enrolled but never completed ACC. She is still an outstanding agent who works hard and has achieved great success as well as tons of loyal clients.
A good cruise agent will not push you to a certain cruise line without bothering to ask what you want in a cruise, what your preferences are, etc.