I’m afraid I worked in Spain a long time ago, the 1980s and 90s. I guess a lot has changed since then and my knowledge is not at all up to date. You also haven’t said what type of course you’re looking for. Is it the certificate in TEFL or a shorter course focused on one topic?
At the time I was living in Spain, International House and British Council offered good courses in Barcelona and Madrid. You could check out those two places. They both have good reputations with many employers but they do have the disadvantage of costing more than other courses.
You are absolutely right to be asking these questions before choosing a course. It is important to know what your job prospects will be before paying for a course. I suggest that before you sign up for a course, you talk to somebody at the training centre about what they do to help trainees find work. You can also ask training centres to put you in touch with people who have completed their courses in order to get first-hand opinions. If the centre is serious, they should be able to do this for you.