If someone uttered words of disbelief or mocked Islam or disbelieved in pillers of faith or disbelieved in Allah and his attributes and powers can I call him a Kafir/disbeliever/atheist?
If they actually have willingly done what you claim then it is permissible to call them kafir/disbeliever as they have committed Kufr and the word falls true on them. It would be in the first case mentioned in the hadith:
أيما امرئ قال لأخيه: يا كافر، فقد باء بها أحدهما، إن كان كما قال، وإلا رجعت عليه
Any man who says to his brother: "O disbeliever," it will apply to one of them. Either it is as he said, otherwise it will come back to him.
Uttering words of disbelief or mocking does nullify Islam, even if done in jest:
قل أبالله وآياته ورسوله كنتم تستهزئون لا تعتذروا قد كفرتم بعد إيمانكم
Say, "Is it Allah and His verses and His Messenger that you were mocking?" Make no excuse; you have disbelieved after your belief.
Assalamualaikum waramatullahi wabarakatuhu
You are not allowed to call someone kafir because only Allah knows the people that believe and the people that disbelieve. Also by calling someone a kafir can also make you too as a kafir, there is an Hadith that one said
Sahih al-Bukhari 6103 Narrated Abu Huraira:
Allah's Messenger (ﷺ) said, "If a man says to his brother, O Kafir (disbeliever)!' Then surely one of them is such (i.e., a Kafir). "
So please my brother don't call someone a kafir