One thing to bear in mind is that very few teachers delivering metal clay training have had any formal teacher training. This doesn't mean they're bad teachers though. The Silver Metal Clay Diploma includes a training module which teaches lesson planning and learning theory. It also requires the trainee teachers to plan and deliver a training session and only trainees who pass this are awarded the full Diploma.
You might want to ask the trainer what formal teacher training they've had before booking a course with them.
Another way to find out the quality of the training is to ask other students who have trained with the teacher. The PMC Guild has a section where students can rate their teacher and give feedback about the standard of training they had. This includes teachers from all over the world and is available to non-members.
Teachers who have websites may have testimonials on them. A good teacher will always ask their students for feedback on the training and this may be available for you to see. Or maybe the teacher would be willing to give you the name of a satisfied student so you can check for yourself.
Also look for a code of teaching practice. The Mid Cornwall School of Jewellery has a code of practice for all their own training plus, their Support Centres have voluntarily signed up to this code of practice so you get peace of mind when booking classes with them. Read more about the training code of practice here