include documentation, old tests, marketing people, conversations with the developers, reading the bug database, trial and error, and writing up presumptions to be presented to developers for evaluation and correction. Again, look for variety and creativity in finding solutions.
QA people need to be creative problem solvers. They like to grab onto a problem and figure out the solution by whatever means they can. They will peek at the answers of a crossword puzzle to break a deadlock. They will come up with a new solution to an old problem. They are aware of the details when finding a solution, yet they have the ability to think outside the box, to appreciate new and experimental aspects. Some successful interviewers keep one or two "brain teaser" types of puzzles on hand for the candidates to work out. Candidates are asked to solve the problem and explain their thinking as they go. Whether they find the answer is not as important. Listen to their thinking process as they work. If they are able to attack the problem from many directions and not give up after the first failures, they are showing the right thinking style. Particularly look to see if they dig into the problem with real enjoyment. A true QA person would.
Of course, QA people need to be intuitively technical. They can usually program a VCR and use most technical equipment without needing the instructions (at least for basic functionality). Other people go to them for help with technical things. Listen for examples of this in their conversation. For example, if they are computer inquisitive, they don't just use software, they tinker with it. They inquire into the details and obscure corners of functionality and try things to see how they work. They may have stories of some creative accomplishment using software differently than intended by the developers, such as using a spreadsheet to write documents.
Good QA people are always learning, whether advancing their technical skills or learning something entirely new. Listen for signs of self-directed learning in the interview.
Good QA people have a sense of ownership and follow-through in their work. They are directly responsible for their work and its contribution to the overall process. They are good at taking a general instruction and fleshing out the details of their work on their own. They will work long and hard at it. Let them tell stories of their achievements and successes in overcoming bad situations. Look for the passion, the ownership, and the pride.
The key thing to remember is that the kinds of skills and mindset needed for QA work is different from those needed for other roles. Spend some time getting to know good QA people in your organization and getting to know what characteristics make them successful. Seek out their opinions on what to look for. Develop a consistent interviewing approach that you use over and over so that you become familiar with the range of responses from various candidates. And for goodness' sake, use your own QA people, even the new ones, to evaluate new candidates.