I've been thinking about this as well. I will be hiring two or three new guys next year. I'm replacing my whole crew except for one. I have had guys come work for us that said they have mowed yards plenty of times, and then first day they can't figure out how to get a trimmer started (which always worries me about whether these guys even realize why there's two gas cans in the truck).
Here's what I have surmised. During the interview process I'm not only going to ask about their previous experience but also to show me. Go out to the shop and tell them to get a trimmer off the rack, fill it with gas, and start it. I figure you can learn alot about a guys experience by watching him fiddle fart around to start a cold two stroke motor.
Still haven't figured out how to get them to show me they can mow in a straight line without going to a property. Unfortunately my shop is surrounded by concrete. :\
Last thing I'll add is I plan on trying to hire college kids next year. One of my questions will probably be how their GPA is looking.