Some TEFL providers proclaim customer satisfaction rates and publish happy camper testimonials. Be wary of these as the trainees, unless they are experienced teachers, are not in a position to judge and it may be just another marketing ploy. If the website is full of writing and grammatical errors and any of the links are broken, that should also send out a warning. The same scrutiny must be applied to course materials.

Course duration, class size and staffing

Any course that is less than say, 120 hours of direct contact with trainers and with about 8 hours of observation and teaching practice, should be approached with caution. And even if your choice of course matches these requirements, remember that this is the start of your journey and you will not be the "complete" teacher until you have had years of experience in the field. Even a decent 4-5 week course is not intended to deliver the finished article; how could it?

The trainer to trainee ratio should about 1:3 or 1:4. So class sizes of say 20 with one trainer should also tell you something about the organisation.

Accreditation for the course

If the course does not mention how it has been accredited then simply avoid this provider. Even when the accreditation body is listed be careful as some of the bodies that are mentioned do not accredit courses but are active in the ESL field organising conferences or seminars for example. If a course provider lists its accrediting body then check to see if that provider is actually listed on the accreditation body's website. Even here there is deception as some TEFL course providers establish their own accrediting bodies! Be extremely cautious about 2 or 4 week courses that offer a diploma at the end. A diploma is normally for experienced teachers with an initial teaching qualification and takes a lot longer. If the TEFL course provider offers merely a Certificate of Attendance, this is not a teaching qualification and will be ignored by reputable employers. Where the accrediting body accredits a vast range of unconnected qualifications and claims links to obscure universities, then it is probably a scam.

Candidates

You need to apply the rules about getting into a good university here. If it's relatively easy to get on the course and there is no screening of candidates beforehand, then you must question the course's validity.

Teacher development

Check to see if the course provider offers a route to further qualifications in the field and if you will get credits for some of the qualifications already completed. If you finish the DELTA for example you can use this towards a Cambridge MA TESOL.

Cambridge ESOL offers a range of teaching awards such as

TKT
TKT:Practical
TKT:KAL
TKT:CLIL
CELTA
CELTYL
ICELT
DELTA
IDLTM
PTLLS
DTE(E)LLS
ADTE(E)LLS

I haven't time to detail any of these so do some research if you are interested. And there are many other things I have not covered. I readily admit I am a fan of Cambridge CELTA and Trinity CertTESOL but I have tried to be objective. I am indebted to Alex Case, and have based this outline on his excellent work.