How can I calculate how much tax will come off my next Commission cheque ($8000.00)?

Go to the Online Payroll Calculator (PDOC) or the payroll deduction tables on the Canada Revenue Agency (CRA) website at http://www.cra-arc.gc.ca/tx/bsnss/tpcs/pyrll/tbls-eng.html. You can enter all of the relevant information and it will tell you how much tax will be deducted. Please note that the actual taxes that are deducted by your employer could be different for a number of reasons; however, you should get pretty close to what they actually deduct. Additionally, remember that the taxes that are withheld are not your actual tax liability. When you file your 2010 T1 in early 2011, your actual tax liability could be more or less depending on your overall income level, deductions and credits for the entire year.