if it's a well established tank, and you are maintaining it well, and don't have any algae issues, fish are healthy, you may not find it necessary to test at all- it's partly a matter of your experience level
i rarely test for anything in fw tanks i setup, i use experience and me eyeballs to let me know what's going on, with the exception of iron, if dosing for plants
having said that- the 'official' recommendation stands to test for all the usual suspects-nitrogen compounds (from ammonia to nitrate), pH, phosphate, and hardness