Conspicuous religious attire would be a good indicator, especially if she refused to remove her burka in a private office. And further questions about religious affiliation. They could be rated on a scale of 1 to 100, where the lower the score the more atheistic, middle score being agnostic, and a high score being devout. The score could consist of 20 true-or-false questions about religious belief, each contributing 5 points to their score if answered true.

Possible questions:

Were your parents religious?
Did you attend a religious school as a child?
Do you attend religious gatherings regularly?
Are you strictly of one particular sect?
Do you support martyrs?
Do you want the laws changed to more favor your religious beliefs?
Do you think religious beliefs are more important than other opinions?
Do you think blasphemers should be executed?
Do you think adulterers should be stoned to executed?
Do you think people of other religions should be executed?

.... et cetera