typedef struct XUserDeviceAssociationChange {  
    APP_LOCAL_DEVICE_ID deviceId;  
    XUserLocalId oldUser;  
    XUserLocalId newUser;  
} XUserDeviceAssociationChange  

XUserDeviceAssociationChange *change; // incoming argument from callback

if(memcmp(&change->deviceId, &XUserNullDeviceId, sizeof(APP_LOCAL_DEVICE_ID) == 0))
{
}

if(memcmp(&change->newUser, &XUserNullUserLocalId, sizeof(XUserLocalId) == 0))
{
}