That is quite a philosophical question. How can we be sure that we decide to take our actions and that they are not pre-determined? And is this question solvable at all? There are some that argue against understanding free will, saying that we may not be able to figure it out as a result of quantum determinism, where our observed actions are derived from a more complex mixture of events.
For example, we can visualise what parts of our brains are active before we complete an action. If we can lots of people to repeat this, we can predict what parts of the brain will light up. Then we can time how long it takes from this lighting up to the point at which we do the action. It appears that many of our decisions are taken at the subconcious level, before our conscious mind does the action. So if you do the action “look up” your brain has already decided to do this action. So do we really have free will if our subconcious is making all our decisions?