You can only do that by asking the right questions and looking for the right answers. This should be the first question you ask of potential Realtors. Experience in a given area is vital to understanding the level of expertise that a Realtor has. Realtors that are well versed in a local area or market will often understand how to price a home in your area. They will also have access to contacts interested in your area and will be known by other Realtors for having experience and inventory in your market.
Past clients can be an excellent source of information.
Ask potential Realtors to get you a list of their last 2 or 3 clients and, if possible, a few clients that they serviced in your neighborhood. Ask the Realtor if it is alright to contact these people. Once you have been given the OK, then give a few of these people a call and ask them whether they were happy with the service and whether they felt that the service they received was good. You can also ask them what they would have changed in their relationship with that Realtor. Don't expect to receive information that is severely negative, however, there can be different levels of approval.