Different types of foods cause different levels of acidity of urine depending on their ash contents. (1, 2)
Some minerals are acid forming (acid-ash): phosphate, chlorine, sulfur. The results of eating foods with a high ratio of such minerals are that they are excreted via urine in order to balance the blood pH. The presence of high acid forming minerals in the urine makes it acidic.
Some minerals are alkaline forming: potassium, calcium and magnesium. Conversely, a diet rich in alkaline minerals will reduce the acidity of the urine, since less acidic substances will be removed through urine. (3)
Urine acidity can be easily measured in the lab by taking a urine sample for minerals or acids or by pH test strips which can be done at home.