You can use /etc/os-release file to get the info about Amazon Linux Version, the machine is running.

1. In case of Amazon 1

   [ec2-user@ip-x-x-x- ~]$ cat /etc/os-release
   NAME="Amazon Linux AMI"
   VERSION="2018.03"
   ID="amzn"
   ID_LIKE="rhel fedora"
   VERSION_ID="2018.03"
   PRETTY_NAME="Amazon Linux AMI 2018.03"
   ANSI_COLOR="0;33"
   CPE_NAME="cpe:/o:amazon:linux:2018.03:ga"
   HOME_URL="http://aws.amazon.com/amazon-linux-ami/"
   

2. In case of Amazon 2

   [ec2-user@x-x-x-x ~]$ cat /etc/system-release
   Amazon Linux release 2.0 (2017.12) LTS Release Candidate
   [ec2-user@fresh-amazon-host ~]$ cat /etc/os-release
   NAME="Amazon Linux"
   VERSION="2.0 (2017.12)"
   ID="amzn"
   ID_LIKE="centos rhel fedora"
   VERSION_ID="2.0"
   PRETTY_NAME="Amazon Linux 2.0 (2017.12) LTS Release Candidate"
   ANSI_COLOR="0;33"
   CPE_NAME="cpe:2.3:o:amazon:amazon_linux:2.0"
   HOME_URL="https://amazonlinux.com/"
   

As you can clearly see the two different versions, mentioned as Amazon Linux and Amazon Linux 2.0

Instead of guessing which version of RHEL a particular distro is based off, just run:

rpm -E %{rhel}

For Amazon Linux 2, this will give you 7.