Do you go to regular classes? Just one teacher or several?
Observe your teachers teaching skills, ask questions to the teacher…ask yourself "is this a good teacher? are the adjustments he/she gives helpful/appropriate? are the demos clear? are verbal instructions clear and helpful? is my teacher teaching me or just showing off? does the teacher answer your questions honestly/clearly? "
If the answers you come up with are positive, then ask that teacher where he/she did the training or if he/she can advise you on where to go, what to read etc.
Also when looking for a TTC choose one that asks for some minimum requirements from the attendees such as at least 3 years regular yoga practice, a regular home practice as well as classes, a reading list to be read before start of course.
Check if everyone passes just because they paid for the course…or is there a final exam which some might fail…will you be tested on your teaching skills as part of the course & exam.
Avoid any TTC that allows people new to yoga to join.