Fir's foliage, when crumbled in your hands, smells like lemons.  Spruce smells like your typical x-mas tree.
And as said above Fir is F for Flat
Spruce is S for Square.  In our area theres only 3 really.  Fraser (which arent that common mostly planted), Balsam, and White.  You will see SOME Dougs but they were planted.  Balsam and Fraser have a REALLY smooth bark, Whites have long flat needles that almost look like a blue spruce. Frasiers are a lil tougher, as they look identical to Blasam, but they are so rare up here, unless planted..................