I'm due a raise and I want to find out how much of it will be taken in taxes? How do I find this out?

Ask your HR department

You can est by what percentage is taken now,

Go to PaycheckCity.com | Web-Based Paycheck Calculators (http://www.paycheckcity.com). It has many useful calculators. If you want to find out what your pay check will be then you can use this link: PaycheckCity.com | Sign On to your account (http://www.paycheckcity.com/netpaycalcreg/netpaycalculator.asp)
First select the year and State and then enter your income and deduction info.