The best teacher not only shares the Dharma,
the best teacher also embodies the Dharma.
Without acquiring some wisdom and knowing true Dharma from false Dharma, trying to practise with great diligence may be like a blind person riding a blind horse. Two dangers of practicing blindly like this are that first, one many fall into the error of believing incorrect Dharma, and second, one may practice incorrect methods.Therefore to truly practice with diligence, one should be guided by a teacher who has the correct understanding of Dharma and who practices the proper methods. Otherwise, without a qualified teacher one would not know how to deal with unusual physical or mental states they may encounter.
How do you know if the teacher is teaching the proper Dharma? First, consider whether the teacher has a correct understanding of the key concepts of Buddhism. This means the teacher should exhibit a clear understanding and acceptance of the law of cause and effect. This teacher would not use their position to benefit himself or herself, and would not do unwholesome deeds because they would know very clearly that to receive wholesome results, one ought to engage in wholesome deeds.
A good teacher of Buddhism also needs to understand the law of conditioned arising, which states that all things arise as a coming together of myriad causes and conditions, and therefore everything is impermanent, without an inherent self. Understanding phenomena in this manner means that one possesses basic Buddhist wisdom. A student can thus use their own understanding of karma and the law of causes and conditions to evaluate whether the teacher or method is in accordance with correct Dharma. A teacher who truly practices according to these laws is a good teacher; it a method encompasses these two ideas, it is a good method.