1. . You thought you were good at math, but your GMAT verbal score is higher than your quant score.
2. You are a native English speaker, but your GMAT verbal score is below the 70th percentile.
3. After studying faithfully for months, you took the GMAT. It did not go as well as you hoped. You studied more, took the GMAT again, and got the same score.
4. Bonus Sign that You May Need a GMAT Class: You have a heavy work schedule / active social life / kids.
http://www.businessbecause.com/news/gmat/2000/three-signs-you-need-gmat-prep