I kept forgetting about the AWS CLI which is quite handy to quickly check if SSM is working on an EC2 instance.
So here it is again for my future-self. Use get-connection-status
to return the connection status of the specified managed instance.
$ aws ssm get-connection-status --target i-092af3b3c1234567
{
"Status": "connected",
"Target": "i-092af3b3c1234567"
}
Reference: