Gather the last pay stub or W-2 you received from your employer.
Find the union dues deduction on your paystub. Union dues are post-tax, voluntary deductions, and should be noted in that section of the paystub, together with deductions for insurance deductions and 401k contributions. Many modern paystubs include not only the amount the the current pay period's deduction, but also the amount deducted year-to-date. Deductions are labeled with text or a code; if your paystub contains only coded identification of deductions and you're not certain which is for union dues, check with your payroll department.
Locate the union dues deducted from your pay during the tax year in Box 14 (Other) on your W-2 form. Note that while it's common practice for employers to provide this information, it's not required by the IRS, which characterizes Box 14 as a place where employers can list financial information they'd like their employees to have. If you don't have a union dues entry in Box 14 of your W-2, you can determine the amount of dues deducted during the year from your paystubs; if you haven't got them, you can contact your local union for the information.