As of April 2015, it is not possible to check a MoneyPak balance online. Not only is it not possible to check a MoneyPak balance online, MoneyPak is no longer sold by Green Dot.
While MoneyPak balances are not available online, balances can be checked by following the instructions found on the back of the card. Generally the instructions ask a card recipient to dial a toll-free number to check the balance.
To retrieve the balance of a standard Green Dot Prepaid card, the command "Bal XXXX" can be texted to 43411, where "XXXX" is replaced with the last four digits of the card.