How do i know if my job expense reimbursements were added to my wages in Box 1 on w2? (they are reported in box 12 code L on my w2)

Your reimbursement from Box 12 Code L of your W-2 are never added to your Box 1 taxable wages. They are "Substantiated employee business expense reimbursements" and are not taxable. Therefore, remember that you can not deduct those same expenses on your return.