I still think that *one* of the finest tests ...

I agree it is one of the most important trait of a good programmer to understand other people's code. One can only be good at programming by writing code and reading other people's (good) code to keep on improving and acquiring knowledge.

If a person is good (and quick) at understanding someone else's code, it is a good indication of him/her being blessed with a good analytical mind, and experience/habit of delving in to code.

Of course, this ability helps when working in a team, but more importantly it is required to understand the existing solutions and their drawbacks/advantages. e.g. If one intends to write a web server or a new VM for a language, it would be helpful to refer and understand the existing open solutions, before starting out.

In short, the ability to understand someone else's code quickly is a definite pointer of a good potential programming ability. Getting a person to write code is the best test most times, but the ability to adapt and learn new language/technology faster is governed highly by ability to read code.