An indicator can show show the pH of a solution when added in small amounts. If the pH is below 7, then it is an acid, if it is above 7, it is an alkali (to test pH, the solution must be dissolved in water, therefore it should be described as an alkali rather than a base), and it's neutral if it is exactly 7. The strength of the acid or alkali increases as the pH reading gets further away from 7. (This is not if an acid or alkali is strong or weak.)