The retirement plan at work is located on your W2 Box 13.
This box has an X if you (or your spouse, if applicable) were an active participant for any part of the year in a qualified retirement plan maintained by the employer.
Whether the money going into the retirement plan is taxable this year depends strictly on how the retirement plan is set up. (Your employer is your best source if you want to determine that. Sometimes this money is considered part of your gross wages and sometimes not. Again, it depends on the plan.)
The main purpose of the X in this box is to notify the IRS that the amount of contributions you can deduct for a traditional IRA (Individual Retirement Account) may be limited because you already have a qualified retirement plan in place.