The process does differ based on what level of caregivers you need and whether they are doing it alone or augmenting family. However, in every state, you can verify any stated credential such as CNA, NAR, RN on line. You can also check state/local criminal records on line and I have weeded out people accordingly. Before you shell out for the record check, Google is a great start and may be your last stop. I have also hired people who had a criminal record after grilling them about particulars.
References alone are not reliable -- colleagues are often getting a piece of the action and patients are -- let's face it -- dependent or sometimes even fearful of an "ex." Ask for an academic reference (preceptor, other faculty) as well as professional ones. Depending on the level of care your dad needs, place postings at local pre-health professions programs (e.g. Pre-Nursing students), nursing schools, general university and CNA program alumni boards, and yes...even Craigslist (if you use the services and/or gigs categories, you can avoid the job posting fees). Network among friends, colleagues, everyone you know. Post on local forums. Lots of people are looking for steady work these days, and it's no knock on them to be looking on their own.
As far as the interview, have a conversation just the way you would if you had met a new colleague at work, teacher, clergyperson, plumber, gardener or whatever. Don't fire a lot of prefab questions off, other than standards like why are you interested and what hours are you available. Just talk and mostly listen. Learn about the person as a person first and a professional second. Use your instincts to know who you (and especially your dad) want to be with. Credentials are not (usually) as important as willingness to learn and ability to care. The group of caregivers I've met who had ALS experience were not any more impressive as a group than those who didn't. There's no magic in that since it's mostly chance. I presume that you'll meet finalists with your dad, and also give top candidates and your dad time alone for their own form of communication, before choices are made.
Some people here are happy with using agencies. You can try that route and see how it goes, but I would not rule out your own abilities before you have taken them for a spin. The experts on your dad are all in your family, not sitting behind an agency desk. And, of course, there can be major differences in cost.
Let us know how it goes...all the best.
--Laurie