Red cabbage solution how do you know its an indicator?

what eveidence could you have that shows that a red cabbage solution is an indicator?
Red cabbage solution how do you know its an indicator?
It changes colour when you add an acid or base.

It can change from red (for very acidic solutions) to greenish (for basic solutions)
Reply:its color changes when you put different things to it - e.g. vinegar (which is acidic) or baking soda (which is basic).