Per IRS Publication 590-A Contributions to Individual Retirement Arrangements (IRAs), starting on page 12:
Limit if Covered by Employer Plan
As discussed earlier, the deduction you can take for contributions made to your traditional IRA depends on whether you or your spouse was covered for any part of the year by an employer retirement plan. Your deduction is also affected by how much income you had and by your filing status. Your deduction may also be affected by social security benefits you received.
Reduced or no deduction. If either you or your spouse was covered by an employer retirement plan, you may be entitled to only a partial (reduced) deduction or no deduction at all, depending on your income and your filing status.
Your deduction begins to decrease (phase out) when your income rises above a certain amount and is eliminated altogether when it reaches a higher amount. These amounts vary depending on your filing status.
To determine if your deduction is subject to the phaseout, you must determine your modified AGI and your filing status, as explained later under Deduction Phaseout. Once you have determined your modified AGI and your filing status, you can use Table 1-2 or Table 1-3 to determine if the phaseout applies.
On page 11:
Generally, you can deduct the lesser of:
However, if you or your spouse was covered by an employer retirement plan, you may not be able to deduct this amount. See Limit if Covered by Employer Plan, later.
Full deduction. If neither you nor your spouse was covered for any part of the year by an employer retirement plan, you can take a deduction for total contributions to one or more of your traditional IRAs of up to the lesser of:
This limit is reduced by any contributions made to a 501(c)(18) plan on your behalf.
Are You Covered by an Employer Plan?
The Form W-2 you receive from your employer has a box used to indicate whether you were covered for the year. The “Retirement plan” box should be checked if you were covered.
Reservists and volunteer firefighters should also see Situations in Which You Aren’t Covered, later.
If you aren’t certain whether you were covered by your employer's retirement plan, you should ask your employer.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.