Typically when people are looking to sell their home, they will get a few of the local agents in to make presentations. Agents are on their "best behaviour" through this process and may well just tell you what you want to hear in terms of pricing.
You want an agent who is going to represent you well, and so the best way of gauging how well they do that is to "secret shop" them, and see them in action!
Visit one of their open homes and watch how they interact with potential buyers. Do they stand around playing on their phone, or do they approach people in a friendly way? Do they just send a junior agent to represent your property? Do they follow you up to see if you are interested in the home afterwards? Do they offer to take down your requirements, and follow you up when they get something suitable?
These are all things a good agent who is working hard for their vendor, will do. There are many agents out there who won't go the extra mile, and who will sell the property for less that it's worth just to churn their inventory.
Selling your home, and choosing the right agent to do it, can make the difference of up to 10% of the property value. Do you want that $$ in the buyers pocket as a "great buy" or in your own pocket, as a seller?
"Secret shopping" and agent is one of the ways of separating the wheat from the chaff and finding a really good agent who is going to get you top dollar for your home.