Before spending hard earned money and a child's time on SAT prep course, find out:
1. If the students are grouped according to ability. I taught several SAT courses for prestigious places and this was not true. When a student taking algebra is lumped into the same class as one taking trigonometry, learning cannot take place. The group is just too diversified and the child aiming for 800 has very different needs than the one aiming for 500.
2. If class size is small. You are spending quite a bit of money so be sure your child is getting plenty of attention. Make sure the people running the group will not just fill the room to capacity.
3. What materials will be used. Make sure the teacher is not just going to do the same stuff your child can do on his own. I taught for a school once that just photocopied Barron's problems and passed them off as their own work.
4. Consider hiring a private tutor instead. You will pay more per hour but will probably be able to accomplish more in less time, making the cost the same and the gain significantly higher.
5. And lastly, make sure your child wants to take the course. I have seen too many kids sitting in these classes, bored to tears and fiddling with phones, waiting for the time to pass. You can't force someone to learn.