How would show that lemon and tomato contains acids?

The acidity of any substance can be tested by a PH paper. 
There are also many chemical methods but these methods only tell you whether the substance is acidic, basic or neutral in nature.
Use Color code system along with PH paper,you can compare the colors and determine the exact acidity.

Note: Acid turns blue litmus paper to red. This is one of the tests for acidity.