Press and hold the Home button until Siri starts. Say “Hey Siri” to start Siri. This requires iOS 9 or later, and the “Hey Siri” feature enabled in the Settings app. To enable this, open the Settings app, tap “General” and then “Siri,” then toggle Allow “Hey Siri” on.
If you only want to check your most recent message, say “Read my latest message. "
If you choose not to reply, Siri will move on to reading the next new message.