Study Instrument Registers
Just as important as knowing the ranges of the instruments that you are writing for you should also have a clear idea about their registers and dynamic possibilities of them in these registers. There are a few charts out there which show one or the other, for example in parts THIS ONE. However this chart doesn’t show how the registers influence the dynamics of the instrument (e.g. it is impossible to play a real forte at the lowest register of the flute etc.).
Most of this knowledge however can be gathered from orchestration books and be put into an own chart. So make sure that you don’t just know the ranges (which can also easily be looked up in case of doubt) but maybe even more importantly how the instruments behave dynamically and timbrally over these ranges.
Be aware that there might be a bit of a gray area there as it is also depending on the player and instrument as well as – to some degree – subjective perception.