Evergreens defoliated for only one year usually die. Repetitive defoliations every year because of root rot, a girdling root, or other root damage can weaken the tree over time, finally killing it. Poor soil conditions, such as backfilling, compacted soil, or waterlogged soils, also weaken the tree. In these cases, leaf drop is gradual, not sudden. Discolored leaves, whether they fall or not, are often a sign of chemical damage.
Short shoots fallen with clusters of leaves attached often indicate animal damage.
Abe Lincoln once said that the height of a tree is best measured when it's down. It so happens that there is and you don't have to buy fancy gadgets to do it. Hold a ruler at arm's length and note how many inches tall the tree appears to be. Record how far away from your eye's the ruler is. Next, measure the distance from where you are standing to the base of the tree you are measuring.