Can we wear black cap? One Palestine friend told that black cap is a Jews symbol and style?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The clothes of men should meet the following conditions:
1- These clothes should not be in imitation of disbelievers' clothes. The evidence is the Hadith reported by Muslim from Abdullah Ibn Umar who said: "The Prophet saw me wearing two clothes dyed in saffron, whereupon he said: 'These are the clothes worn by non-believers, so do not wear them" .
2- The clothes should not be tight hence exposing the Awrah.
3- The clothes should not be transparent, uncovering the Awrah properly.
4- The clothes should not be in imitation of female clothes. Abu Dawood and al-Nasai reported from Abu Huraira that he said: "The Prophet cursed the man who wears the garment of the woman and the woman who wears the garment of the man."
So, any clothes that meet the above conditions can be worn by men, otherwise they cannot.
It should be noted here that the clothes of disbelievers that are forbidden are those which are peculiar (unique) to them, not those which are used by others as well.
Allah knows best.