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

No File Exists
Check below answers in case you are looking for other related questions:
Ruling on greeting women with salaam and returning their greeting
Supplication (du‘aa’) upon entering the house

No File Exists
Related Answers:
Ruling on shaking hands with the opposite sex
Is it bid’ah for a person who is already there to say salaam to one who has just arrived?
Ruling on giving and returning salaams in public washrooms
Ruling on greeting women with salaam and returning their greeting
Supplication (du‘aa’) upon entering the house

Recommended answers for you:
What can benefit the deceased after his death? Can he hear what the living say?
The punishment for zina (fornication, adultery) and how to keep oneself from going back to it
Why do we pray Zuhr and ‘Asr silently?
Ruling on playing chess
Women cutting their hair and removing facial hair