If the tax credit reduces your tax liability to 0 then any remaining credit is not used and does not carry forward to the next tax year.
Review your federal tax return, Form 1040 - The credit will shown on Line 20
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.