Dynamic fitness tests measuring aerobic fitness, muscular strength, muscular endurance, and flexibility. The tests are only as accurate as you make them, so reading instructions carefully, and performing them often, should bring more accurate results.
Although the tests are only likely to highlight what you already suspect, such as a bodybuilder achieving "excellent" muscular strength test results, or a marathon runner displaying an "excellent" score for their aerobic fitness tests, it can be used for trend analysis, which can identify progression or decline and might, in turn, alter the way you train your client. Also, it'd be ignorant to assume you've taken all factors into account. Although unlikely, sometimes the test results can throw up a surpise, such as a powerlifter with poor grip strength.
A PAR-Q and Static Fitness Tests should be completed prior to performing Dynamic Fitness Tests. Information on both can be found on GymProfessor.com.