The tax on your taxable income is taken from the IRS tax tables depending on the type of income you have reported on your tax return.
Your taxable income is on Line 15 of the Form 1040 and the tax on the taxable income is on Line 16.
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.