get the currently installed SSM Agent version from within the operating system

Run the following command for Amazon Linux and Amazon Linux 2.

yum info amazon-ssm-agent

On Ubuntu

$ sudo snap info amazon-ssm-agent
name:      amazon-ssm-agent
summary:   Agent to enable remote management of your Amazon EC2 instance configuration
publisher: Amazon Web Services (aws✓)
store-url: https://snapcraft.io/amazon-ssm-agent
license:   unset
description: |
  The SSM Agent runs on EC2 instances and enables you to quickly and easily
  execute remote commands or scripts against one or more instances. The agent
  uses SSM documents. When you execute a command, the agent on the instance
  processes the document and configures the instance as specified. Currently,
  the SSM Agent and Run Command enable you to quickly run Shell scripts on an
  instance using the AWS-RunShellScript SSM document.
commands:
  - amazon-ssm-agent.ssm-cli
services:
  amazon-ssm-agent: simple, enabled, active
snap-id:      T09mpujiTnzSdSCuqNkE7YXXTWDq13tC
tracking:     latest/stable/ubuntu-20.04
refresh-date: 6 days ago, at 22:56 UTC
channels:
  latest/stable:    2.3.714.0 2020-03-03 (1566) 18MB classic
  latest/candidate: 2.3.930.0 2020-03-20 (1935) 29MB classic
  latest/beta:      ↑
  latest/edge:      ↑
installed:          2.3.714.0            (1566) 18MB classic