How do I know if I should file my 1098 with my taxes if my employer paid for my school expenses as well as a Pell grant

The amount that was added to your W-2 can be used by you to claim a tuition credit.  Since you paid tax on that amount, that much was paid with "your money", not your employer's money. 

There's even a "loop hole" to allow you to count some of the Pell Grant for the tuition credit.

. The student reports all his scholarship, up to the amount needed to claim the American Opportunity Credit (AOC), as income on his return. That way, the parents  (or himself, if he is not a dependent) can claim the tuition credit on their return. They can do this because that much tuition was no longer paid by "tax free" scholarship.  You cannot do this if the school’s billing statement specifically shows the scholarships being applied to tuition or if the conditions of the grant are that it be used to pay for qualified expenses.

Using an example (example does not consider the tax free employer benefit): Student has $10,000 in box 5 of the 1098-T and $8000 in box 1. At first glance he/she has $2000 of taxable income and nobody can claim the American opportunity credit. But if she reports $6000 as income on her return, the parents can claim $4000 of qualified expenses on their return.

Books and computers are also qualifying expenses for the AOC. So, extending the example, the student had another $1000 in expenses for those course materials, paid out of pocket, she would only need to report $5000 of taxable scholarship income, instead of $6000.