In the Elite Call Center (built into CM), you can assign either an Auto-In or Manual-In button. These are your "ready" buttons. The "Not Ready" is the AUX-Work button. The light next to the button will light Green when the feature is active. So an Agent would be "ready" if their Auto-in/Manual-In button was Green, and "Not Ready" if the Aux-Work button was green.
If you have something else then you're probably not going to get any visual indicator on the phone, especially if you are logging in via a web page or other device.