Look at your tax return. Verify the income and taxes withheld, then follow the math
You can view your Form 1040 at any time using the online editions. Click on Tax Tools on the left side of the online program screen. Click on Tools. Click on View Tax Summary. Click on Preview my 1040 on the left side of the screen.
Did you have any income they didn't take withholding out of? Like interest or dividends or 1099 income? If you only have W2 income you might need to increase your withholding.