When receiving Per-Diem, is that amount included on the W-2? If so, how is it deducted?

If your per-diem has been included as taxable wages, then you claim a full deduction of your per diem on Form 2106.  The deduction is limited by 2% of your AGI and itemized deductions.  However, you do not reduce the per diem by the amount of "reimbursement" because your per diem is being paid for with taxable income.  Thus, you may deduct the amount (up to the Federal daily limit for the area in question, as found on gsa.gov) on that form without reducing the amount of the expense, and TurboTax will determine how much of the deduction will be used to reduce your income.  Please see this FAQ and embedded links for additional information:  https://ttlc.intuit.com/replies/3300445

Beginning in 2018, unreimbursed employee expenses are no longer eligible for a tax deduction on your federal tax return however, some states such as California continue to provide a deduction on your state tax return if you qualify.

Only 4 groups qualify on IRS

1. Armed Forces

2.Qualifed performing artist

3. Fee base state or local official

4. Employee with impairment 

https://turbotax.intuit.com/tax-tips/jobs-and-career/employees-can-deduct-workplace-expenses/L58LFjAPa