Both MyTaxExpress/T2Express have a "Validate" feature to help you detect some common errors in your return. It is recommended that you run validation before filing your returns. If the return can't pass the validation, the software will not proceed to do any electronic filing.
In the newer versions of myTaxExpress, simply click on the Validate icon at the top of the software window. A Validation Result window will pop up for you to examine. If you are doing a coupled return you can switch between the results of the couple by clicking on their name.
To start the validation process on older software versions, simply press the icon "Validate" at the top of the software window. An Error window will pop up to display errors detected. You can correct those errors accordingly, then repeat this process until no errors are found.