Pointing mistakes without saying Lashon Hara

How to act? when somebody says not wise things.... like lecturer or professor that contradicts Tora or tsaddikim ... shall I accept it? keep silence?

ב"ה Shalom, I would like to help you more but to give an accurate answer, I need a specific situation. Therefore, I will give general guidelines. If the speaker is a gentile, although we have a general mitzvah of endearing G-d and to propagate the belief in one G-d, ( Rambam sefer Hamitzvot 3) it is not our obligation to correct the beliefs of other people. If there are Jewish listeners to the lecturer, you may want to focus on them and try to show them the correct Jewish view. From the academic aspect, if the lecturer is open to discussion you can challenge his or her ideas, but it should be done in a respectful manner and you should be well prepared to present your Jewish view. A poorly presented argument may have a reverse effect. If the circumstances of expressing you views are not in your favor, Our Rabbis in the Talmud (Yevamot 65b) gave us a rule, that just as there is mitzvah to say something that will be listened to, there is also a mitzvah NOT to say that which will not be listened to. If the speaker is Jewish and propagating ideas which contradict the Torah, our attitude towards the lecturer should be the same way we deal with any another Jew who is distant from Torah observance. This essentially means, to find a way to draw the person closer to his/her identification with the Jewish people and closer to belief in Torah and Mitzvot. As Hillel already said in the Mishna in Pirkei Avot, a person should make himself a student of Aaron who loved peace and pursued peace... and drew people closer to the torah".) Avot 1:12) This is not achieved by means of an argument. The mitzvah of bringing Jewish people back to their Jewish roots and to be knowledgeable of their heritage is found all over the Talmud. You therefore have to use your good judgment. If there is a contradiction to Torah, of course you cannot accept it, but sometimes there is no choice but to be silent. Also, if something specific arises, such as something scientific you may want to hear the Jewish explanation of things . Not it all cases will things be contradictory. Good luck and all the best.