I imagine this varies from tradition to tradition. In my tradition, the Thai version of Mahasi Sayadaw's teaching, anyone can pass on the teachings, so in a sense anyone can be a teacher. What this means is that there is no governing body going to reprimand you if you go out and teach others as you have been taught. This seems in line with the Buddha's way himself; the only requirements as far as being a teacher is concerned relate to fulfilling a role as a monastic teacher (i.e. regarding monastic discipline).
That being said, to become recognized as a teacher in my tradition, one generally has to undergo training both theoretically and practically - and of course, it goes without saying that one must have previously completed basic training in insight meditation (at least one month of intensive practice, preferably more).
Theoretically, one must have studied and/or been taught about the sixteen stages of insight knowledge as found in the commentaries and Visuddhimagga, along with the sort of "symptoms" to look for in someone going through each of the stages. A basic knowledge of Theravada Buddhism (usually through the Thai Dhamma exams) is also understood as a given.
Practically, one would be required to first listen in on another teacher's interviews with meditators (I spent a year doing this, every day for about five hours including ceremonies) and then eventually teach yourself under the guidance of a qualified teacher.
There is generally no certificate given, though there is a standard course in Thailand now for monks that lasts 40 days and gets one a certificate of sorts. Since there is very little practical experience in teaching involved, it doesn't seem quite complete as a teacher training, more of a basic requirement that one should then use to undertake further practical study of the art of teaching.