How to know if I am using Rails 2.3 in production?

I have a client that is running a Linux VPS with a Ruby on Rails site. He said is a Rails 5 but I enter in the Linux VPS and type rails -v and I get rails 2.3.14. There is a way to prove that the site is not running rails 2.3.14? Something like to see if have turbolinks for example. Because turbolinks not exist in rails 2.3.14. Something that you can see it just looking at the web page. Some tip please.