If the battery is empty and that is why the car does not start, we will easily understand it, from the car instrument display.
In our attempt to start it by turning the start switch, we will see the instrument screen dimming or not even being able to complete the start. If it is completely empty it is very likely that we will not see any indication on the instrument display when we turn the switch to the starting position.
Another way is to check the battery voltage with a voltmeter or a special meter. If it is fully charged it should be about 12.7 V at standstill. If it is completely empty it will be below 12.00 V, around 11.7 V. The car is difficult to start with the battery voltage below 12.30 V.