Which feature would allow a quick isolation of an issue with iCloud services?

The System Status Page is the most effective feature for quickly isolating issues with iCloud services. This page provides real-time information about the status of various Apple services, including iCloud. When users experience problems, they can check the System Status Page to see if iCloud services are down or experiencing disruptions. This resource helps to identify if an issue is related to Apple's infrastructure rather than a user-specific problem, allowing for a more efficient troubleshooting process.

Other features such as iCloud Keychain, iCloud Storage Management, and iCloud Sharing are designed for specific functions related to managing passwords, storage usage, and shared files respectively. While they are essential for the overall functionality of iCloud, they do not offer the broader diagnostic capability that the System Status Page provides for recognizing service-wide issues. Therefore, relying on the System Status Page is the most logical step in pinpointing and resolving service-related concerns.