I've configured the PHPMailer plugin to how I think must be correct. But I have no way of knowing if it actually works. I'm not magically getting e-mail from it, so either it's not sending anything or it doesn't work. I can't know which is it.

I need some way of sending a test e-mail through zenphoto's e-mail handling stuff, or some way of triggering an e-mail send with 100% certainty, so I can at least get some idea of how well I configured PHPMailer.

So in short: what do I do to test PHPMailer config?

Try the contact_form plugin .If you are testing locally,mail sending may not be working at all.

The Comment Form plugin also has a email notification option. So you can just try test comments.

Getting PHPMailer to work from [i]localhost[/i] is a pain, especially if you try to use the Gmail SMTP servers.(it only allows mail from registered domains or via a complicated registration of your IP address.
https://support.google.com/a/answer/176600)

I got it working by checking my ISP's (KPN) SMTP server addresses and then figure out which one worked.
Only the non-secure configuration turned out to do the trick.