Predicting events is no proof. You might even just "guess" things. It's a matter of probability, but you get my point. I don't know the "acid test".
Let's assume it's possible, but not freely available in terms of "moving freely" in time. Might as well be dangerous (for your health, the universe, ...it's a risk) or you have to make sure certain people won't be able to find out it's possible. Yeah yeah conspiracy, whatever, there are mean people who just like to sabotage. Let's say you can go back in time, but you're still bound to the timeline. There might be more restrictions.