Is a big check from a Canada company American taxable...?

Depends.....If the Canadian company does business in the US, and the income is issued by a subsidiary in the US, it is taxable in the US. For example, if you worked for Toyota in a US plant, you don't pay taxes to Japan. BUT, if the Canadian government is demanding taxes on the income, you can claim taxes paid to Canada as a credit or as an itemized deduction. Usually the credit will work better. As a US resident, this income will be taxable to you in the US as well. That is why you get the credit so you wont be double taxed.