I've been a babysitter on an online site, and it is annoying to interview for a short or infrequent job. At the same time, I wouldn't hire someone without it, so it's tricky.

Call them, tell them what sort of work you're offering, see if they don't mind "dropping by" with their first aid quals and police check (maybe a written reference?) Tell them it will be 20 minutes or less, and let them pick the time. That way you both get to suss each other out.

An alternative (and my personal fave method) is get them to come an hour early for a date you don't *have* to keep. Spend that time introducing the sitter to the kids and if you get the heebie jeebies, just can the outing. I love this because I've never been turned away. A weirdo might not be too pleased though...