How Do I Know Which Events Are Adjustable?

i really want to be able to see at a glance which of my events are theoretically movable. So say something comes up, you know, i've planned a to do this thing. But then work sends me nemel and says, hey, actually, are you available for meeting? I know it's come late in the day. If i look at my calendar, then i can see, ok, actually, this is not a fixed block. And yet, i don't really have that many not adjustable events ecause i really try hard not to have meetings.