There is no way of knowing if a student is good at teaching.
It probably is a sad truth that some teachers don't know their subject. I have met quite a few that don't, or can't be bothered.
But to educate means to draw out, not to put in. It's not all about subject knowledge, it's about getting the best from a student, identifying their weaknesses.
Anyone on here who has done a pgce knows that it's bloody hard, it's intense, it is soul destroying. You learn how to plan a lesson, how to differentiate, how to meet learning needs, different learning styles, socratic questioning, to meet assessment objectives, how to identify progress by the end of the lesson. Student teachers have regular observations throughout their training. The feedback can be brutal.
Once the pgce is completed, teachers have an NQT year. More observations, more feedback, more training. Continual CPD, continual updates from the exam board. It takes 5 years before a teacher is really experienced in the job. By which time, they are either looking for promotion or looking to leave.
The best tutors will not only have plenty of experience, they will also have been examiners and will have worked closely with the exam board. That is another hard grind.