Sign in into the Text Integration website: https://secure.textintegration.com/
On the Dashboard in the Subscription Information box, you will see the number of text messages you have used up under 'Text Sent.' The number of 'Texts Sent' noted will be from the start date of the subscription until the time you logged in.
Note: The number reflected here is the number of text parts you've sent and not the number of text messages.
eg. You sent 1 text containing 3 parts.(1 text part is 160 characters). The number that will reflect in your Subscription Information - Text Sent is 3.
More information on SMS Concatenation here.