Can you “formally” determine one’s religion and beliefs? You can certainly establish some criteria; perhaps even phrase a nice questionnaire. In modern times, you can also carefully monitor brain activity. Still, you can never really tell what’s on a person’s mind.
Furthermore, people have a nasty tendency to change their minds. Many times concerning small things, but once in a while – changing their whole system of values and beliefs. The traffic of the faith goes in all possible directions. People become religious, and people become secular. People convert from one religion to another, or convert between sub-flavors of the same religion. You may even find those who made it both ways in a single lifetime. Some became religious and then secular again; others (though it seems less frequent) go both ways in the other order. Everything can happen and everything does happen.
In August 2006, militants in the Palestinian Gaza Strip freed two kidnapped journalists from the American Fox News channel, after forcing them to presumably convert to Islam, by reciting the so-called Shahadah: “There is no god but Allah, and Muhammad is his messenger”. Did it turn those journalists into good Muslims? Though some Muslim scholars list more conditions of the Shahadah, the act itself was good enough in the eyes of the kidnappers.
So how can you really check if someone is a “Muslim”? On a similar note, what’s the meaning of the “religion” section in the Israeli database of residents?
The answer is probably that we classify people according to our classification system, not theirs.