You can see the use of your tax card by opening the last payment confirmation, you can find it under My Pages TR and there under my documents. Once you have opened the confirmation, you can see the use of a tax card by going down to "Tax Information" where you can see how high tax rate you are using.

If you want to register or change the rate of your tax card (skattkort) or your personal tax discount (persónuafsláttur), you can do so on My pages TR under Enquiries.