There are measures you can use to determine the effectiveness of a tutor. Essentially, you should evaluate them based on both their academic effectiveness and their personality match with your student. Before employing them, it is always recommended that you take some time to follow up with references or testimonials from previous employers. It is also a good idea to review the prospective tutor's resume prior to hiring.
Once they have started working with your student, there are two ways to get a measure on how they're doing. First, an effective tutor should work with you and your student to set some attainable academic goals. After a few sessions, check in with the tutor to determine your student's growth, and their progress toward the goals. If they are having trouble moving your student forward, consider re-addressing the goals you have set, then spend some time investigating their methods. If a student is unable to demonstrate progress after four or five sessions, it may be best to explore other options.
In the end, however, the value of student feedback should not be underestimated. Students rarely learn from individuals they don't like or that give the impression they don't care. Likewise, most students are very sensitive to situations where their time is being wasted. After a few tutoring sessions with a new tutor, take some time to listen to your student. Ask them a few open-ended questions, such as "What do you like about...?" or, "What have you learned so far…?" If their responses are mostly positive or enthusiastic, then it is safe to assume a productive relationship with the tutor is in the works. On the other hand, if your student expresses feelings of frustration or boredom, the tutor may not be a good match for your student.