They will typically know the oncologist on a professional level. They will have also had reports from the patients they have sent in the past on the quality of care that they received. Comprehensive Cancer Centers typically have a team approach to the treatment of malignancies. You may consider a consultation at such a facility, and ask to meet an entire team of physicians there. You may want to speak with people that have a similar diagnosis and have already gone through treatment, and then ask them for a recommendation.
Nurses can be a great resource.
Most have contacts with other nurses who may work with oncologists. Sometimes local support groups can be helpful. You may find people who have already experienced a similar cancer diagnosis. The Internet can be a great resource to obtain information and locate facilities in your area. Just remember, many excellent private practice oncologists do not have their own website, so this resource should be used in conjunction with the previous recommendations.