This is an old question. When the Buddha was walking through India preaching, he came to the country of the Kalamas. They asked him this question, (paraphrasing): “A lot of preachers come to preach to us. Each preacher tells us that he alone is right and all the other preachers are wrong. How do we know which preachers are really right?”
The Buddha replied:
“Yes, Kalamas, it is proper that you have doubt, that you have perplexity, for a doubt has arisen in a matter which is doubtful. Now, look you Kalamas, do not be led by reports, or tradition, or hearsay. Be not led by the authority of religious texts, nor by mere logic or inference, nor by considering appearances, nor by the delight in speculative opinions, nor by seeming possibilities, nor by the idea ‘this is our teacher’. Kalamas, when you yourselves know: ‘These things are bad; these things are blameable; these things are censured by the wise; undertaken and observed, these things lead to harm and ill,’ abandon them. … Kalamas, when you yourselves know: ‘These things are good; these things are not blameable; these things are praised by the wise; undertaken and observed, these things lead to benefit and happiness,’ enter on and abide in them.”
– Kalama sutta, Anguttara Nikaya, 3.65
Or, in short, “By their fruits shall you know them.”
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