You can use "Message Alert Notification". The information below was taken from the R8 manual - it explains how it works and how to program it. It works great.
Message Alert Notification
Use this feature to program a button on your PARTNER telephone to see the status of another extension’s message light. To use this feature, you must have an intercom Auto Dial button button programmed for the extension you want to monitor. When Message Alert Notification is “on,” you will see the status of the message light for every intercom Auto Dial button programmed on your telephone.
Considerations
Message Alert Notification
• Must be programmed on a button with lights on a system phone.
• Can be turned on and off at any time, even during a call.
• Must be used with the intercom Auto Dial feature.
When Message Alert Notification is “on,” the intercom Auto Dial buttons and Manual Signaling buttons display the status of the message light at the corresponding extensions. A blinking red light indicates that the message light is lit and the extension is idle.
After system restarts, there may be a slight delay in updating the status of the message light at the other extension(s). There may also be a momentary delay in turning off the message light when all the new messages are reviewed in the extension’s voice mailbox.
Message Alert Notification also works with the PARTNER-CA48 Intercom Auto-dialer.
Programming
To program a Message Alert Notification button:
1. Press Feature 00.
2. Press a programmable button with lights.
3. Press Feature 27.
4. Program another button for this extension, or exit programming mode.
Using Message Alert Notification
From a system telephone, press the programmed button to turn Message Alert Notification on. Press it again to turn it off. Use the button like a switch—when the light is on, Message Alert Notification is on.