One of the surprising things about Google+ is that there’s no search box. You can search for people, but you can’t search for content of any kind. It’s expected that Google is working on a pretty sweet search feature, likely combining elements of #hashtags in it, but no one knows for sure.
- site:plus.google.com Post by Firstname Lastname keyword
- site:plus.google.com inurl:posts/* keyword
Not all of the public posts are indexed yet, but it’s a great place to start. Hopefully a proper search feature will be added soon.