Exam preparation courses such as TOEFL should be taught by teachers with a specific experience in that field. I’ve worked for several language schools, mainly in Milan, Italy, where you were instructed to teach a course that you were not familiar with. Since I told the management that I had experience preparing students for IELTS, they assumed I could also do TOEFL. I told them that this was an entirely different format and that I was not familiar with it and that I did not want to do that course. The director was vexed and consequently gave me less work. Reason enough to leave that school!
You have to be able to help your students with the format: how to answer, what kind of questions to expect, the timing, etc. It is almost like a workshop that takes a practical approach to preparing for the English test.
But also from a purely didactical point of view, you have to be familiar with the test to advise your student what to focus on:
Listening techniques, reading and writing techniques and practice, speaking interview techniques and practice, grammar and vocabulary revision.
I now work in Belgium, for the English Academy
, where I teach IELTS prep. courses, next to general English. I have two American colleagues who are specialised in TOEFL and TOEIC and three other teachers are specialised in the ESOL test courses. That is the way it should be.