You take your card to the store and they can check it, or you give them your phone number and they can check it that way. It's also on your receipt when you buy something. (1,000 points=$1.) You can also check online if your email is enrolled. If you don't have a card, you can't redeem your points unless you know your PIN number, which you can check online. If you do have the card, you bring it to the store and the cashier can redeem them. I work there, that's how I know. :p
To check the balance on your Kmart card you need to visit your nearest Kmart store and hand over the card to the clerk at the store. The clerk will then scan your card and the details of your account will then be shown. Alternatively, if you are a member of the Kmart website, you can log in to your account and view your balance under the Account at a Glance section.