How to review Excel Models for errors?

I have to review a bunch of god-forsaken, unwieldly 50-tab models from client companies relatively often. A few tips:

1. Start at the output: If the final output is an income statement, I’ll start there and trace back until I reach hardcodes. That tells me the basic logic of the model. From there you can look across the rows and columns to figure out any discrepancies in formulas.
2. Pay attention to the little green corners on some cells: These denote inconsistencies in formulas, so always audit these to see if there are issues.
3. Ctrl+F for “.xls”: This helps you find external links that might be broken or incorrectly linking to another excel file because someone haphazardly copy and pasted a sheet from another excel file.
4. Do manual re-calculations to check common line items: If a line for “revenue” appears across multiple tabs, make check rows comparing this across tabs and investigate discrepancies.
5. Read any in-cell comments (the red corners of cells): May have helpful notes from prior users.
6. Compare outputs in powerpoint: If it is a model frequently updated, output the old version on a powerpoint. Then, output the new version on another slide. Quickly flip back and forth to see changes and investigate.