How do we determine the reactivity of the halogen elements?
How else but by experiment........?
But luckily, you don't have to do these experiments. Fluorine and astatine are both halogens, and are good reducing agents. But fluorine is far more reactive. In fact, fluorine is the most reactive element on the Periodic Table. Should you meet a fluorine chemist, he may likely have eyes or fingers missing due to the hazardous nature of fluorine reactivity.