of claiming and saying that someone is polytheist 'Kafir"? or "Takfir" someone? I mean if we see someone doing shirk or polytheism?

The principle in the actions of a Muslim is that they must be interpreted in a manner that does not challenge his monotheism. It is impermissible to accuse him or her of disbelief and polytheism because his Islam is strong evidence to his belief. This is a general rule that all Muslims must follow when judging the actions of their Muslim brothers. Malik, the imam of Madina (may Allah have mercy on him), said: "Whoever commits an act that carries 99 aspects of disbelief and one aspect of faith, he is still a believer".

The Kharijites are the only ones who deviate from this established ruling and work on deluding Muslims and taking them out of Islam. Abdullah ibn 'Umar (may Allah be pleased with him) said: "They [Kharijites] took verses which were revealed about the unbelievers and then applied them to the believers." [Bukhari].