The replaceability test
Ask what it would take to rebuild one module. If the answer involves changing three others, the boundary is nominal. Real modularity shows up as an ability to replace a part while the rest of the estate keeps operating.
Observe each module separately
When each module reports its own throughput, errors and latency, a failure can be attributed quickly. Aggregate-only monitoring tells you that something is wrong without telling you where, which is the expensive part of an incident.