Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Answering your question, `Abderrahman Ibn Nasser Al-Barrak, a famous Saudi scholar, states:
If there is no sign on her to indicate that she was a Muslim, then the ruling should be according to the town in which they found her. If most of the people of the town are Muslim, then she is a Muslim, otherwise, the funeral prayer should not be offered for her.
Allah Almighty knows best.