Writing software well (or even reading well existing software ... which is a must, when someone needs to learn, or just maintain old code), seems to require a combination of cognitive skills. That combination can be detected, even when a person has no previous programming experience: http://www.eis.mdx.ac.uk/research/PhDArea/saeed/ .

From the linked homepage:

    >> We (Saeed Dehnadi, Richard Bornat) have discovered a test 
    >> which divides programming sheep from non-programming goats. 
    >> This test predicts ability to program with very high accuracy 
    >> before the subjects have ever seen a program or a programming language.

Edit: added the note about reading software.