How do you know whether an issue is infrastructure or application?

Core Summary

The fastest way to split the two is to look at the signal: if error rate is spiking but latency is fine and volume is normal, it is almost certainly application (a bug, a missing resource, a permission). If latency is up, error rate is flat or proportional to latency, and one resource is saturated, it is infrastructure. The shape of the metrics tells the story.