Get recommendations for beekeepers in your area from a local university extension office. You're looking for beekeepers who are also classified as bee swarm catchers.
Ask the beekeeper about removal methods before hiring her. Generally, beekeepers will not be harming the bees because they want to keep them to create new hives. They should also remove the swarm at night when the bees are less active, not during the day.
Find out whether the beekeeper is planning to charge you for the service of removing the swarm of bees from your tree. Hire a beekeeper who does not charge; plenty do not, since they will be benefiting by keeping the swarm of bees.
Look into the credentials of the beekeeper before hiring him officially to remove your swarm. The beekeeper should have experience removing similar swarms of bees, such as a swarm of honeybees, and the experience should be in your general climate and area.