"how does one know when supports are needed if not included in the original model?" - Intuition

Seriously, the best way is just step thru the layers and make sure nothing is printing in mid air.

if you just imagine printing the object from the build plate up, you'll get to recognize most times when supports are necessary.

You can also just use the Auto Supports to see what FlashPrint recommends.

Also, under Supports, you can choose between tree like Branch Supports and Linear/Pillar type supports.

You might want to print the model using each type of support to see how they work and better understand the differences.

Enjoy!