I would suggest reviving the old idea of a quick quiz, and it doesn't necessarily have to be done online; the organizer can simply arrange a meeting time with each participant and ask them tricky questions, and then post the answers in the thread.
I would suggest reviving the old idea of a quick quiz, and it doesn't necessarily have to be done online; the organizer can simply arrange a meeting time with each participant and ask them tricky questions, and then post the answers in the thread.