Deductive thinking is based on logic and the facts. A good detective can use deductive reasoning to find a killer.
If you want to know how to be deductive, read a Sherlock Holmes book or watch the show "House" (about a clever doctor who is based on Holmes). Both Holmes and House are masters of looking at the facts — which are often confusing — and making deductive conclusions. Thinking deductively involves critical thinking and careful attention to detail, not guessing or making things up. That's why being deductive is one of the best ways to crack a case.