Many employees will simply tell you anything you would like to know. Learn from this and make certain your employees do not give out confidential information. A good policy is to determine what sorts of information you want to give out and make that part of your business process. Build your intelligence over time. If it takes you several months to slowly piece things together you end up with valuable information.
Information goes stale quickly in today's business environment.
Strategy is very important. Strategy is usually the most important thing you can learn about a competitor. Details are sometimes hard to find but the overall strategy is often easy to determine. If your voice and face are known consider using someone else to do the investigating. Remember no one knows your mission but you.