There are 2 main parts to any application. The UI which is what the customer can see and provide feedback on, and the underlying code which they really can't know if it is healthy or not.
Therefore it is important to conduct a 'test please' on the internal code and architecture of the application.
Ideally conduct a small 'Code + Architecture Review' for every Sprint. Assuming a 2 week Sprint, schedule a 4 hour (2 architects x 2 hours) review.
The following are items that are addressed in an architecture/code review:
Background information/overview of the project
Current system
Objectives of the system
Number of users (internal, external, edit, read only)
Current technologies
Current environment (SOA, SOE)
Points for discussion
Rich client
Web client
Smart client (any disconnected users?)
Technology choices
Requirements for 'package' software
PerformancePoint
Reporting Services
Accounting packages
SharePoint
Usage Telemetry
Performance Monitoring
Data migrations
Data reporting
User experience
Network
Responsibilities/players
Infrastructure
Deployment
Disaster recovery
Change control/source control
Build Server
Performance
Scalability
Extensibility
Design patterns
Maintainability
Reliability (failover servers?)
'Sellability' i.e. is the solution appropriate for the client?
Note: If you are using Enterprise Architect, be aware of technical debt:
Add a datetime of the last time the diagram was modified so we have an indication of when it is out of date
On your diagrams, be aware that some parts are done and some are not.