You will definitely hear the fan roar over the engine roar. The fan is 3 times louder than the engine. And you will also "feel" the airflow as it blows a tornado worth of air when its engaged. It will follow abrupt rpm changes.
A disengaged fan will be quiet as it literally be freewheeling and wont change rpm when you rev it up.
Its not a very technical test, Its infact the simplest test. If you think you are not able to judge the fan on idle rpm then you can manually increase engine rpm to where you can test. If you can literally hear the fan at all rpm right from start to all driving rpm and do not experience the fan going quiet a minute after start. Your fan is faulty (wrong oil, too much oil, or badly adjusted BMS on it)