W-2: Box 12 code G amount & Retirement box (13) not checked. Am I covered by a retirement plan at work to calculate the deductibility of my Traditional IRA contributions?

My AGI is over the amount needed to claim a traditional IRA deduction on my return if I am considered covered by a retirement plan at work, but I can take the IRA deduction if I am not considered covered by a retirement plan at work.  I have a Box 12 amount on my W-2 (code G - 457(b) plan) and again the Retirement plan in box 13 is not checked.  So I am confused if I am considered covered by a retirement plan at work?  I would like to make a deductible IRA contribution if possible.