Argos issue two types of gift card. To check the balance use the 0345 number written on the back of the card, or the new online balance checker.
Plastic/Card gift card: Bought either in-store or online from Argos, identified by their plastic or card construction, a card number that begins 10000, and a scratch panel to reveal a PIN allowing on-line use. They expire 3 years from purchase and cannot be extended, though some cards last over 10 years. They often have an expiry date written on them though the actual expiry date is much longer; check online. They can be spent in-store and online, though only up to 3 cards can be used in an online transaction. Alternatively reserve online, then pay in-store with your gift card. Gift cards cannot be reloaded. Using a gift card in-store will result in a new card being issued with any remaining balance, whereas using online will just leave the balance on the same card.
E-gift card: Bought online from the Argos website only, these also begin with 10000 however they expire after only 2 years. You can check the expiry date using the balance checkers.