Once the item has been used, every character, including the one purchasing the commendation, has their reputation gain increased by 100%, so a fail-safe way to see if your account has a certain faction's Grand Commendation running already is to do a daily quest. Having made your way to revered, it's possible that you're fairly aware of the amount of reputation usually awarded by a certain daily quest.
If you're not sure exactly how much reputation a quest usually awards, there are resources available to help, such as wowhead's database, which can be used to check the reputation reward of any daily quest. There are also addons like Factionizer which can help in-game with reputation rewards.
Once you've established the reputation reward usually given by a daily quest, it's easy to see if your account has had a 100% increase in reputation applied, the reward will simply be doubled. And don't worry, as Bashiok says, Community Manager Rygarius is working on this with the development team.
Mists of Pandaria is here! The level cap has been raised to 90, many players have returned to Azeroth, and pet battles are taking the world by storm. Keep an eye out for all of the latest news, and check out our comprehensive guide to Mists of Pandaria for everything you'll ever need to know.