But i think there are a few teachers around who's behaviour is impeccable and you can't go wrong with by following their advice.
They've been around long enough to trust that their advice is sound and comes from a good understanding of the teaching.
Moreover you can see the qualities of such a teacher reflected in the serious student.
If a student is serious and follows with dedication and to the letter the advice of his teacher but in the end there is no developing of any kind then i guess there is something wrong with the teacher.
Let me first state that I fully I agree with what you are saying. Nevertheless there's a comment I would like to make about the bold (by me) portion of your sentence. Not everyone access their status correctly in terms of seriousness or dedication; adding to that, there's always the problem of interpretation. While a student may think he is following his teacher advice to the letter, there might be the case that the teachings and advice were misinterpreted unintentionally. This may lead the student to project blames that the teacher hasn't in fact. How to counter this? Look at others and see how they are going. Usually a good teacher also has a handful of good students (not students that brag, which is a different matter all together). So it is also helpful to check if everyone is stuck or there are people progressing. Although this too is not very easy to access, you must use your best guess.
Last, but perhaps more important, usually if you are serious and committed, not suffering from any serious psychological unbalance, not gullible or easily impressionable, are very careful about getting the teachings right and making a real effort to study while being diligent in your practice, if your teacher is good you will know it beyond any shadow of doubt. You will know it because you will make progresses noticeable to yourself. In some cases, like when there's direct introduction involved, mandatory in Dzogchen, the moment it happens you know your teacher is an unsurpassable treasure and any doubt will vanish for good, never to come back. Just be sure to confirm you recognized instant presence and not some transitory mental state of bliss or other thing (that will let you down later on). If you did, and a sign of it is what I'll say next, it will be like a paradigm shift. A game changer if you prefer. A full stop in your doubts about your lama. You met your root teacher and it was not a choice or a decision you made, but an occurrence that left no remaining doubt. So all those questions about good teacher/ bad teacher and so on vanish. This should be our aim, knowing that we too may need to put some effort so that we have the required conditions do do it, as few are fortunate enough to get it without any preparation.