View the application's app.xml file.
The app.xml file is located in the application's WEB-INF directory.
Examine the registrationInfo tag in the app.xml file.
For a legacy application, the registrationInfo tag is a version 2.x. For example, registrationInfo version="2.2.4".
For a current application, the version in the registrationInfo tag is at least 3.0. For example, registrationInfo version="3.0".