Bands of stomates in 2 rows on the lower portion of the leaf.
First, determine that it's a fir and not a spruce. Once you a certain its a fir, you have to look at how leaves are arranged on the stems and avoid the reproductive one. Look for the presence or obscene of stomatal bands on the upper surfaces, they are always on the lower surface, not the upper for this species. Look up for cones and down for cone scales and bracts.
Not at all easily. I used my trusted Pacific Coast Trees by McMinn and Maino.
Foliage is found in one plane, often called 2 ranked but I like the plane concept, or in 1 dimension. To use a key on these you have to find the resin glands with are inside the leaf, either along the margins or near the main vein. These are shallow and along the margin. To see the resin glands you need a 10X hand lens. Cut the leaf in half crosswise and then carefully squeeze the leaf with your nail while looking for where some resin comes out.