Lemon is more acidic, it has a pH value around 2.0
meanwhile, orange has a pH of around 3.0
watermelon is the least acidic, it has a pH value of around 9.0, which is an alkali.
Besides testing for the pH value with universal indicators, we can do simple taste test (only food!). usually the more sour a food is, the more acidic it is. lemon is the most sour food out of three, and it's the most acidic.