That's a great question, and I must admit it concerns me greatly as the course director of a non-Cambridge/Trinity certificed TEFL cert course!
I have two main comments: First of all, there are other certificate validating organisations out there who are definitely worth their salt, but as you obviously appreciate, some have better 'names' than others. The one I use is called IATQUO, which is run by the retired-head of TEFL at Trinity (!) which means that the quality is absolutely but they don't ask a prohibitive price just to 'buy' a well-known name, for example. Which in turn allows our school to produce a good, seriously validated course, for a more reasonable price.
The other vital point, which is exactly the same for teacher training courses and English lessons alike, is that the actual trainer / teacher who runs the course and gives the input sessions is perhaps the most important factor governing whether or not the course will be a success. Some of the larger teacher training organisations won't be able to tell you who your trainer will be, and may even object to your asking, as if you were questioning their ability to supply top-quality trainers for all their courses. Or they may simply not feel like taking the time so it's up to you to use your intuition to decide if you are going to trust them with your future four weeks, and one of the best ways of doing this is the contact you have with the school as you are going through the application process.
That'll do for now! Good luck in your future English teaching career - it's a great vocation!
Sab Will, Will Power English