It is not uncommon for e-filed returns or paper filed returns that claim a credit for US tax for CRA to request verification.
US tax paid per Canadian withholding slips are not requested because the slips are issued by a Canadian entity. They are looking to US tax claimed per US reporting slips such as W2s, 1099s, 1042s or from the US 1040/1040NR tax returns. US social security tax and medicare (ie., FICA) is creditable if it relates to US source wages or services performed in the US which is evident on the W2.
Foreign tax credits claimed in respect of US computed from the US tax return based on determination of what portion of the tax payable of the return relates to US source income that is creditable is usually supportable by submitting to CRA your schedules and copy of the federal (and state where applicable) return. Recently CRA has Read more