When do you think you know a tool enough? With the current rate of revisions, it might never happen, but what is the stage when you feel comfortable with a new tool? I like working with new tools, especially because I think I should use the tool suitable for the need, not for me. However, the challenge in that is to understand the tool before using it for work.
With so much information available on the Net it has been quite easy to find out the features and the capabilities of a tool. But they tell only part of the story. My understanding of the tool has increased when I have been able to find out what it cannot do or when does it not work. It has increased further if I have been able to find out why it cannot do something. This information is not easily found, unless you try the tool. Or at the most if you follow someone who knows it well enough to write or talk about it. No wonder most of the incapabilities are usually documented in personal blogs out of personal experiences!
This leads me to think that another important question to ask while interviewing someone about a tool, is its disadvantages and incapabilities. Only experience can answer this satisfactorily. Or that person has at least followed people who have tried it.
When I try to learn something new, whether a programming language, a framework, a tool or any piece of software, I make sure that I try to find out about the limitations, cons, incapabilities too. There is no one-size-fits-all solution. While learning a tool it is important to learn when to not use it as well. What do you think?