There are some substances in the middle, which are not acids or alkalis. We call these neutral substances. Water and Salt are two examples. It is impossible to tell whether something is an acid, alkali or neutral just by looking at it. You have to test it with a chemical.
Some indicators are very simple.
An example is blue litmus paper - which is paper with a special chemical soaked into it. If you dip it in some alkali it stays blue , and if you dip it into neutral it also stays blue . However, when you dip it into an acid it will change to red. Another example is phenolphthalein , which is a clear liquid. If you add some to an acid or to a neutral solution it stays clear.