Three referrals should be enough to assess the quality of work as well as the communication skills of the painter. Ask about the overall quality of the work, the work style of the painter, how the work has held up over time, and if they would hire him or her again. Be wary of any professional that refuses to give you references; a quality painter should be happy to have to speak to past clients.
The right paint stroke to use in interior painting is highly debatable. It's not a talent as much as a skill that is learned through practice. Many experts paint in a “W” pattern when using foam rollers, but others simply roll up and down then sideways with either brushes or rollers. The motion isn’t as important as making sure that the application is even and drip-free across the entire area. However, technique is important when you're covering over wall repairs.