Tumor response rate is measured using imaging techniques like CT scans, MRI, and PET scans. The most commonly used criteria are the RECIST (Response Evaluation Criteria in Solid Tumors) guidelines, which define how to measure the size of tumors and classify the response. Complete response indicates the disappearance of all target lesions, while partial response involves at least a 30% decrease in the sum of the diameters of target lesions.