Answer every one you can.
When I spot a "Take our survey" link on a Web site or feedback URL on a store receipt, I generally answer the questionnaire. Sometimes it's a smooth, well orchestrated experience. Occasionally I find myself wondering "What were they thinking?"
Just as an editor reading a manuscript watches for grammar, clarity, flow, and other issues, there are a number of aspects you'll want to watch for when taking a survey. While some of the issues I list here are an abstract analysis of the survey, the most important thing to pay attention to is your personal reactions to the questionnaire. After all, you are a respondent, and what you feel while completing the survey will be happening for other people as well. When you get started you'll only notice a few issues with each questionnaire, but over time the evaluation will become second nature—just like an advertising executive dissecting Super Bowl commercials.