Question Summary:
How do I know if someone is a Muslim

Question Detail:

my question is, how do i know if someone is a muslim, for example my aunt told me about a muslimah sister looking to get married, should i assume that her family are muslims, her extended family are muslims as well as her grandfather and uncles etc
note, my aunt did not mention anything to say if she is a revert or not or that her family are non muslims, if you could ensure you give a very detailed answer, as this is only a snippet of what i'm going through. i have OCD aswell
also how do i know if someone is a muslim or not [outside when i see people] should i assume anyone asian/arab/tirkish with beard is a muslim or every woman i see with a hijaab or niqaab is a muslimah, when i see a white man with a beard like that of a muslim, should i assume he is a muslim even though he does not wear a thobe

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Your issue is simple.
Request your aunt to find out more about the Muslimah in reference and her family. How do you expect another Muslim to regard you as a Muslim, regard the next person as a Muslim in the same way.
And Allah Ta’āla Knows Best
Ridhwan Ur Rahman
Student Darul Iftaa
Cardiff, Wales, UK 
Checked and Approved by,
Mufti Ebrahim Desai.