It would be very difficult for an employer to falsify a W-2. Why don't you have check stubs? I suspect Biff is right: that $100 in "taxes" probably included federal, state, FICA and Medicare, and maybe some others depending on where you live.
Without a check stub it's hard to say if the W-2 is right, and impossible to prove fraud. If you know your hourly rate and the number of hours you work per pay period you could enter that information into a paycheck calculator to see how much of each kind of tax would be taken each check.You could also ask the company to provide you a pay-period-by-pay-period accounting of your gross-to-net wages (essentially a printout of the information on your check stubs). Given how you parted ways, they may turn you down, but it's worth asking.
Very possible that only $50 was withheld for federal income tax. There are other taxes that were also withheld that could have made up most of the $100 per check.
do you have paystubs that show a different amount? if it was a prt time job, sure it's possible. a big chunk of those taxes were social security and medicare taxes and state WH which is not refundable
