Chronic kidney disease is generally diagnosed by measuring the serum creatinine at least twice over a three month period. Using the age, race and gender one can calculate the glomerular filtration rate, and use this number to classify the disease into 5 stages.
Kidney disease may also be diagnosed in the presence of proteinuria or other "markers" such as an abnormal ultrasound.
For further infromation refer to the website http://ckd-epi.com. It will give you the mathematical formula and more details about the classification system. Also one can visit either the National Kidney Foundation http://kideney.org or the KDIGO http://kdigo.org websites.