Coming short of grace (Hebrews 12:15)

Hebrews 12:15
See to it that no one comes short of the grace of God; that no root of bitterness springing up causes trouble, and by it many be defiled...  (NASB)

Exercise foresight and be on the watch to look [after one another], to see that no one falls back from and fails to secure God's grace (His unmerited favor and spiritual blessing)... (Amplified)

Looking diligently lest any man fail... (KJV)

"See to it."

"Be on the watch." 

"Looking diligently." 

Obviously coming "short of the grace of God" must be visible.  That is, there are signs that it is happening. And it must be visible before the bitterness happens. 

What are the signs of coming short of or falling back from grace?