Apart from the fact that it is Allah (swt) who guides people and Who (finally) judges them on the Day of Judgement; what concerns us here is only which people we can be sure of being káfir or disbelievers.
That said we only know a person being káfir by his attitude towards Islam. If he is an active enemy, he is káfir. Otherwise you cannot judge about it, because it is only God who knows what is in the hearts.
For an in-depth treatment of this issue see ”Who Are The Disbelievers?”
”Some of The Main Arguments From Sh. Hamza Yusuf’s Exposition”.
A káfir is someone who directs its hostile stance against Islam or the Muslims, either with his hand, or with his tongue or his pen. He is a káfir, whether he belongs to any religion or not.
The definition is from the Qur’an, it refers to those who claim that they belong to people of the Book, but who do not respect Islam or Muslims. In the Qur’an: Those who drive you from your home (Sura 22-40), and those who are fighting against your religion, these people will be seen as kuffár. Those who respect Islam and the Muslims and do not fight them, they should be respected equally.
Some people do not agree with some things from Islam, if they express their disapproval in a rational, respectful manner, it can be discussed and explained, no harm involved.
Those who do not follow the Prophet ( sallAllahu `aleihi wa sallam ), or have a ”flawed” faith (`aqídah), the Qur’an does not call káfir [ - except if they plainly and openly associate partners to God], because they belong to people of the Book. Then they have their rights, they should be able to practice their religion, their possessions and also their honor will have to be respected, and their women can be married, all of that makes the people of the Book not to be considered káfir. [Then finally Allah shall judge.]
They are not perceived as káfir because they have rights with the Qur’an, but once someone takes up arms, or the (evil) word or the (lying) pen against Islam and the Muslims, he will lose these rights.
Káfir: the covering over (hiding, detracting from) the truth, ie. they know that it is the truth, but they do not want to admit it. They are those who have been told the truth but they deny it, so they are called Káfir (Sura 2-6).
Káfir can also mean ‘godless’ (Sura 4-151), it depends on the context. The critical limit is his (or her) position towards Islam, either it is a positive attitude or it is a negative attitude.
It follows that if a non-Muslim treats Muslims in a normal and proper manner, he should not be considered as Káfir, also because it can not be said that he has got correct knowledge about Islam, while he is exposed to the anti-Islam propaganda in the West.
But Allah knows most and best!