UnityDetectHeadset is a Unity native plugin which enable to detect if a headphone is plugged on iOS & Android.
To use it, import the Plugins folder and inside your code call: DetectHeadset.Detect();.
Note it will return true on non supported platform.
For Android, you need to add permissions:
<uses-permission android:name="android.permission.MODIFY_AUDIO_SETTINGS"/>
<uses-permission android:name="android.permission.BLUETOOTH"/>