If you contribute to a 401(k) or similar plan through your employer, it should be noted in Box 12 of your W-2. If it's not there, you should contact your employer to find out the reason and potentially get a Corrected W-2. Contributions to qualified retirement plans usually deducted using pre-taxed dollars. However, if you contributed to a Roth IRA, there is no tax deduction for that.
