I’m afraid I have to disagree with Antonio and his idea that the quality of a course is somehow reflected in the amount of money a course provider spends on advertising.
If you really want to weigh one course against another start by asking for their course syllabus and compare it to others. Check their tutors’ credentials. Make sure the course requires some actual work on your part and it is not based on “Yes or No” answers or “Read This Unit and Summarize It” type of coursework. Try and find out what other students who have taken the course say about it. Basically do as much research as you can.
You could start by taking a look at TEFLWatch.org. This is a site where TESL/TEFL courses and course providers are independently reviewed and rated.
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