You’ve probably never dealt with a volunteer teaching agency, let alone one that is based here in Thailand. You need some peace of mind from actual people in your home area. Medical students, engineering students, lawyers, nurses and police officers in your home country will answer your questions and share their experience with you. Most of them were first time volunteers, as you possibly are.   You can contact them directly and confidentially.    If an agency says that cannot provide you with references, you might ask them why that is.  

You wouldn't want to end up like some volunteers you read about on internet forums.  They're dropped off at a school in the middle of nowwhere with just a bare floor to sleep on in a very rundown school.  Not only can the experience be uncomfortable, in some cases it can be downright dangerous.