When CodeDeploy is not running:
ps -ax | grep "codedeploy-agent: InstanceAgent" will return:

 7443 pts/0    S+     0:00 grep codedeploy-agent: InstanceAgent

When CodeDeploy is running:
ps -ax | grep "codedeploy-agent: InstanceAgent" will return:

 7679 pts/0    Sl     0:00 codedeploy-agent: InstanceAgent::Plugins::CodeDeployPlugin::CommandPoller of master 7675
 7756 pts/0    S+     0:00 grep --color=auto codedeploy-agent: InstanceAgent

And in either case the test at https://github.com/continuousphp/aws-codedeploy-agent/blob/master/test/integration/aws-codedeploy-agent/serverspec/default_spec.rb#L10 will assert true.

A better test would be one which filters out the grep process and only returns the CodeDeploy process if running:

ps -ax | grep "codedeploy-agent: InstanceAgent" | grep -v grep