The vital role of Formula Auditing. An amazing but little known tool in Excel.
Trick 1) F2 Key, Shows formula in current cell (rather than looking up at the Formula Bar)
Trick 2) Ctrl ~, Show all formulas in worksheet (~ is located below the Escape key)
Trick 3) Formula Auditing, draw arrows to formulas (Add to your Quick Access Bar)
See Examples on this Video