What are some different ways/tools to verify that keep-alive is working on the server from the client's end?
You could try
and look via top, if 5 workers are constantly serving the requests keepalive should work (-k) switch. Do the same without -k and see the difference.
As Ron Garrity says, you can use Curl like this:
And it outputs these two lines if keep-alive is working:
And if keep-alive is not working, then it just outputs this line: