There is no API to determine the current signal strength of the phone radio on a Windows Mobile Phone Device. But as the signal value changes, its updated in the registry at the following location.
“Signal Strength Raw” value under the above key stores signal strength value in percentage (0 – 100).
You might want to know in your application when signal strength status changes. Polling registry is a resource intensive operation and should be avoided. Instead you can use RegistryNotifyCallback function provided by State and Notifications Broker mechanism. This powerful feature is available since WM 5.0 and gives applications an easy way to get notified when a particular registry value changes. Read more about it in MSDN.