If it is not known whether a person is a Muslim or a kaafir, can we say salaam to him?

It was reported in the hadeeth that you should say
salaam to those you know and those you do not

(narrated by al-Bukhaari, 12; al-Fath
1/55) but this applies only to Muslims, or those who appear to be Muslim.
It was also reported that it is forbidden to say salaam to Jews and Christians, because
the Prophet
(peace and blessings of Allaah be upon him) said: “Do not initiate the
greeting of salaam to a Jew or Christian, and if you meet them in the street, push them to
the narrowest part of the road.” (Narrated by Muslim, 2167). He
also said: “If the People of the Book greet you with salaam, say ‘wa
‘alaykum’ (and also upon you).” (Narrated by al-Bukhaari, 6258).

But at that time the People of the Book were distinct
from the Muslims in their dress and appearance, and they were not allowed to resemble
Muslims. But in these times, unfortunately, many Muslims look like them, and we can no
longer tell the difference between a Muslim and a Christian. Everyone – except for those
whom Allaah wills – looks the same in dress, in being clean-shaven, in wearing
nothing on their heads, or wearing western-style caps, so the matter is more confusing. If
someone who looks like the mushrikeen says salaam to you, say “Wa
‘alaykum,” and do not initiate the greeting, because of the uncertainty about
him. If he objects and tells you off, then apologize to him. You had reason to act as you
did, because you did know whether he was a Muslim or a Christian, because he is not
dressing as a Muslim and prefers the dress of the Christians and others. Tell him that
“whoever imitates a people is one of them”

(Saheeh, narrated by Imaam
Ahmad, 2/50-92), and advise him to distinguish himself from the kuffaar and
to dress as the Muslims dress, like his father, grandfathers and the scholars of the
Muslims. If he persists in what he is doing, this means that he likes the characteristics
of the Christians and is imitating them, and that he despises the Muslims and is going
against them, even though he does not gain anything from that besides blind imitation.
This indicates that he admires those kuffaar and thinks that their worldly achievements
and inventions, etc., stem from their false religion. This is going too far, for the
Muslims are wiser and more able to invent and produce, so he should not be deceived by the
Mushrikeen.