Blood chemistry. High level of BUN and creatinine with low level of bicarbonate (sodium bicarbonate) strongly suggest poor renal function.
Blood tests usually. A physician can do some simple blood tests such as measure the BUN and creatinine to assess the functioning of the kidney. Once reduced renal function is found, more complex testing is then done that may include scan to look at the size and appearance of the kidneys, a variety of more detailed blood tests and analysis of the urine itself. Often including 24 hour urine collections.