Here's how I evaluate initiative in an interview:
Behavioral interview questions
I'd ask, "Tell me a time when you took initiative." or "Tell me a time you volunteered for a project that everyone else thought was dull or boring."
Most candidates would give an example of a task their boss or some other person asked them to do. This is not initiative. It requires some careful listening and follow-up questions to determine whether or not the candidate was truly proactive.
I would also ask, "Tell me a time when you were given a project without guidance. How did you figure out what to do?"
Here I would evaluate how thorough they assessed different options before deciding on a course of action. If there was only 1-2 options that may not indicative of a truly independent individual.
Before the interview, I'd tell the candidate to do some pre-interview homework. For instance, "Re-design our website and bring mock-ups to the interview." or "Take a look at the Google Maps API, and hack together a mobile app."
Based on their effort, you can evaluate their initiative.
The truly proactive ones would likely do this anyway without prompting. ;)