Here, I found it.
http://www.dance-forums.com/showthread.php?p=710180#post710180
My criteria.
1. The instructor has to be a great tango dancer and a milonguero/a.
2. He/she has an extensive knowledge of Argentine music and culture (in my book, actually, that is included in #1).
3. He/she has already produced a number of adequate social tango dancers.
4. In his/her classes he/she stresses the basics rather than teaching patterns.
5. He/she explicitly encourages his/her students to aspire to all of the above.
What he/she should not be:
-A ballroom dance instructor/show dancer who has never set foot to a milonga. That means:
-Anyone who does not regularly dance tango socially is not considered.
- How many students attend the classes is not a criterion. Some instructors seem to attract crowds of perpetual beginners, walking horrors, crazy and unpleasant people. I believe there is a reason for that.
Yes, teaching tango is a business, but I will not go to those who primarily interested in money, and who ends up encouraging mediocrity and lack of etiquette in their students.
Of course these are only general guidelines that could be elaborated into details and red flags. For example, one who never talks about line of dance in class, who never uses the music to demonstrate the material, probably does not teach social tango for milongas, based on musical improvisation, hence, if that is what you wish to learn, that teacher is not good for you.