Light switch puzzle

I came across this puzzle:

You and 5 unfortunates are [to be] locked up [somewhere]. You all know that there is a single toggle switch initially in the UP position[,] to be found in a special room. Prisoners are kept apart and cannot communicate once the [period of imprisonment] starts. Prisoners are taken from their solitary cells one by one and one at a time, in no [particular] order, to this control room.

The only way out... is if one of you declares correctly that all of you have visited the room at least once. The downside is that you will [all] stay there [forever] if the statement when uttered is made incorrectly. You have a few moments together before the [period of imprisonment] starts[,] to set up a strategy. No one else may interfere with the room. How can you arrange for one of you to be in a position to know that 'we have all of us visited the room at least once'?

See if you can do it. Be honest. Don't try looking up the answer... anywhere (unless you're giving up). Encouragement: I got there - after being baffled by it for a while.