There are two ways to tell if a payment has been declined. First you can run the Declined Payments report on the Reports tab which will give you a list of the declined payments for the time range that you have selected.
Second, you can go to the patient's account and select the Prior Payments tab. Once you do this, if there is a declined payment, it will appear as a red row. This will include details about the decline code and the description for that code. This will look like this: