I used to take interviews for frontend developers for my company. The skill of interpreting the frontend developers is not by difficult programming questions but by listening their different views about any complex page design (say, tell me your views about The Times of India website, what you really think they are missing here??).
I feel the frontend developers skillset should include the design part more than the algorithms and programming.
Apart from basic logical questions, he/she should be asked the questions regarding the integration of frontend with backend, different kinds of libraries/frameworks he/she worked with and what are the pros/cons of using them. He/she should be evaluated by giving some design scenarios which anyone would face while coding frontend. Also he/she should be aware about the importance of simplicity in user interface.
There are no perfect answers to these questions but these questions helps in evaluating the thinking power of a developer when he is given the time to think out of the box.