"Hire slow, fire fast."
Many companies do the opposite. Especially startups. Pressure for speed and growth forces managers to be fast at hiring ("we need someone on customer service position YESTERDAY!") but very slow at letting poorly performing people go. Don't make this mistake.
We came up with these interview questions over the years of running our own customer service team and developing a help desk app in close contact with our clients - support teams in over 2000 companies all over the world. Please note, that most of the questions are meant for technology companies and software startups, some of them might not fit other industries.
Now, to the actual questions.
I don't have to tell you why is this important. Also, it's OK to be annoyed by a customer. As long as you're not showing it. And as long as it does not not affect your work. Also, ask the candidate WHAT was the most annoying about that client specifically.
What drew their attention to customer service industry? What are their career goals? Your objective here is really to understand if they treat this a temp job. If they do - oh that's a bad sign. If they don't - embrace it and tell them, that helpdesk agents in your company are not some "second grade" employees, they get invited to all the important meetings and get involved into all the product decisions.
They are, right?
99% of customer support is done via email anyway, so writing is crucial. No need to test it during the actual interview, but do keep this in mind while exchanging emails and text messages prior to the actual meeting. Short clear and systematized writing is a great sign.
"Clear writing is a sign of clear thinking. Great writers know how to communicate. They make things easy to understand. They can put themselves in someone else’s shoes. They know what to omit. And those are qualities you want in any candidate."
Jason Fried, 37-signals
Go through your case history, pull some previous tickets and send a couple of them to the candidate, even before inviting them to an interview. Ask them to provide a reply. And you will quickly see if they're a good fit.
Having no previous experience in your industry is OK. But they still need to be "tech savvy". They'll learn the rest from their peers. But do test their ability to find things on Google. This might sound funny, but sometimes you just need to copy-paste the error-message from a customer into the search field and find a workaround.
Things might go south. Actually, things will go south. Servers will crash, unit-tests will fail to catch a bug in your code, your app will be DDoS'ed, your hard-drive will crash and you will lose your backups. The important thing is to be open and transparent about the problem and while developers and sysadmins will stay up all night, solving the problem, your customer service agents will be the ones who'll be getting all the customer "love" during those hard times. They did nothing wrong but they take all the blame.