I'm a one person operation most of the time. I have a couple of photographer friends who I occasionally call upon when I need help. One is a professional photog, but makes his money from from landscapes and architecture, not people. The other is a long time amateur/semi-pro who has a real job, and isn't planning on leaving it. I love hiring these guys because I know they know what they are doing, and they'll probably never be my competition.
If I was hiring an assistant for wedding photography I'd just assume they were doing it because they wanted to do it for a business at some point. I live in a college town, so my strategy would be to hire a college student who'd be moving away in 4 years to be somebody else's competition.
I'd expect them to have the technical aspects of running the camera and the flash pretty well figured out. I don't mind teaching someone how I go about a wedding shoot, but they should have the fundamental photography skills mastered. They need to be able to run the camera and the flash in manual, and get consistantly good exposures. At least good enough for print film or raw files.
Being a wedding assistant isn't even half taking photographs, mostly it's hauling gear, setting up gear, taking down gear, holding gear, and guarding gear. I need to be able to tell them how I want cameras or lighting set up, and they get it the first time. Most of the training is going to be on the job, in front of the client; it doesn't look good to have to explain something more than once, and we just don't have time.
Skills in customer service, retail sales, or working with the public would be a bonus.
I'd want to see a portfolio of at least 12 prints, mostly with people as subjects. This is going to determine who I'd hire. Obviously they need to be an asset to my business, and create photos that please the client. I wouldn't mind so much that they will be potential competition someday, because if they have the skills and the eyes, it would seem to be inevitable with or without my help.