Ruling on one who dies and it is not known whether he was a Muslim or a kaafir

If a man or woman is found dead and it is not known whether
he was a Muslim or a kaafir, then in this case distinguishing signs should
be looked for if possible, such as circumcision in a land where the
non-Muslims do not get circumcised, or the type of clothing, or hair dye
(e.g. henna), or any sign by which it is possible to find out whether the
person was a Muslim or not. If there is no such sign, then the land in which
the person died should be taken into account. If it is a Muslim land, then
he is deemed to be Muslim and he should be washed (ghusl) and the funeral
prayer offered for him. If it is a non-Muslim land, then he should not be
washed and the funeral prayer should not be offered for him, because the
basic principle is that whoever is in a land is one of its people and comes
under the same ruling as them so long as there is no evidence to the
contrary. 

This is also indicated in a report that is proven in
Saheeh al-Bukhaari (3013) and Saheeh Muslim (1745) from Ibn
‘Abbaas (may Allah be pleased with him) from al-Sa‘b ibn Jaththaamah (may
Allah be pleased with him) who said: The Prophet (blessings and peace of
Allah be upon him) passed by me in al-Liwa’ or Waddaan and he was asked
whether it was permissible to attack the mushrikeen with the probability of
exposing their women and children to danger. He said: “They are of them.”  

Ibn Qudaamah said in al-Mughni (3/478): If a person is
found dead and it is not known whether he was a Muslim or a kaafir, they
should look for signs such as circumcision, clothing and hair dye. If there
are no distinguishing signs and he is in a Muslim country, then he should be
washed and the funeral prayer should be offered for him. If he is in a
non-Muslim country, then he should not be washed and the funeral prayer
should not be offered for him. This was stated by Imam Ahmad because the
basic principle is that whoever is in a land is one of its people and comes
under the same rulings as them so long as there is no evidence to the
contrary. End quote. 

Al-Kasaani said in Badaa’i‘ al-Sanaa’i‘ (7/104): The ways in
which a person is deemed to be a believer are three: documentary evidence,
proofs and implication… With regard to a person being ruled to be Muslim by
means of implication, a child may be ruled to be a Muslim by implication if
his parents are Muslim, whether he has reached maturity or not. He may also
be deemed to be a Muslim because of the land in which he lives. … End
quote. 

i.e., if he is found in a Muslim land then he is deemed to be
a Muslim; if he is found in a non-Muslim land then he is deemed to be a
kaafir. 

And Allah knows best.