To find out if the diet lemonade contains any sugar, Liam should follow these steps (using Benedict's solution):
1. Take a sample of the diet lemonade and pour it into a test tube.
2. Add a few drops of Benedict's solution to the test tube with the diet lemonade sample.
3. Mix it to make it homogeneous.
4. Take the test tube to a water bath.
5. Heat the test tube in the water bath.
6. If the solution (sample of diet lemonade + drops of Benedict's solution) turns orange-red, the diet lemonade contains sugar.