To view how much Global Credit you have, head into Settings > Usage within the app. Global Credit will automatically be applied to any LevelUp purchase.
To view how much credit you currently have at a business, head into 'Places.' The credit you have at each location (business specific + Global Credit) is listed to the right of each business name.
After each purchase you make, the credit amounts under 'Places' will refresh to show the new amount that you'll save, meaning the amount of credit that would be applied if you make a purchase there. The total credit available at each location includes both business specific credit as well as any Global Credit.
Typically, this means that if you claim $5 of Global Credit, you'll see all of the credits under 'Places' increase by $5. Then, if you spend $2 of that $5, the amounts of credit displayed will only be $3 higher than they were originally.