Choose a difficult application
A logo sheet can make almost any identity look coherent. Test the system where content changes, space is constrained and the audience needs to act.
For a cultural brand that might be an event listing with five artists, three venues and a late change. For a product company it might be a release announcement that needs to explain a feature and show the interface.
Look for decisions, not decoration
The test should reveal whether hierarchy, type, colour and imagery provide enough guidance. Any rule that only works in the presentation deck needs another pass.