A question I like asking when running meetups is, "do you think you could explain how to do this to someone else?" I usually ask it just after explaining the rules or steps for whatever we're doing this meetup. This timing only works for a meetup with an explicit activity, though you can also ask attendees if they feel they could explain how to run a dinner party or other less explicit meetup. Often people nod yes, and then I point at one of them and ask them to explain it, and point out some missed details. This is partially a good way to make sure your audience actually understands well enough to do the activity, but I have a sneaky ulterior motive: every person who understands how to run meetups is one step closer to being a meetup organizer.
You have now read the basic point of this post. If you want to read on, cool, lets talk implementation details for a bit.
I suggest not spending too long on this in any single meetup. It's not really worth the cost in pacing to have every attendee explain how to do this until they can do it without leaving anything out or adding unnecessary steps. I usually pick one person and have them explain things once, give corrections once, and then move on. Maybe I'll do two people, each explaining it once. Think of "could you explain how to do this to someone else?" as a small habit you take five minutes for every meetup, like light stretching.
Sometimes their explanation is actually better than mine. It's not at all uncommon that there's some neat analogy or example the use that I didn't think of. When that happens, I copy it with gleeful joy. Take notes! You presumably have thought about how to do this more than they have, but you also knew what you wanted to happen before you explained anything. They have an advantage in that they are just learning it now.
Speaking of which, take note of where they get things wrong. Especially if you're running the same kind of meetups repeatedly, this gives you an excellent opportunity to notice which parts of your explanation people get stuck on or which parts of a technique are tricky to learn. Refine your explanations accordingly.
This question usually applies to what I call the payload of a meetup. If you're running something from the Meetup In A Box sequence like Calibration Trivia, then a good response indicates that the person can run Calibration Trivia once all the people are gathered. It doesn't tell you whether they know how to do all the other steps like announcing the meetup via the local mailing list equivalent or finding a good venue or arranging snacks. Most of this isn't difficult to do, but people often get stuck on announcements in particular. Two ways to get around this are to offer to do the announcing for someone if they're ready to run the payload, and to take the time once in a while to walk through those other setup steps you usually do.
Speaking of payloads, some meetups don't have an explicit activity at their heart. Things like socials and dinner parties don't have formal, written down rules and steps. (Yet, growth mindset.) That doesn't mean you can't share parts of what goes into them. The dinner party version of this goes
Adam: "Those enchiladas were delicious!"
Bella: "Thank you! They're pretty cheap and easy to do in bulk, do you want the recipe?"
[several weeks later]
Adam: "Hey, I'm having a dinner party on Friday; I'm taking a try at the enchilada recipe you gave me!"
I basically had that conversation, the enchiladas are delicious, and they're now one of my favourite food choices for a crowd of vegans.
Some payloads are basically self documenting and really anyone could do it. Some payloads rely on a difficult skill that's much easier to witness and appreciate than to do yourself. If you can participate in a Book Swap, you can almost certainly run a Book Swap. If you can participate in Petrov Day, you can probably run Petrov Day, though you might need to buy some candles. If you can participate in Winter Solstice, you may be a couple years of musical training away from being able to run Winter Solstice.
"Do you think you could explain how to do this to someone else" and "do you think you could run this meetup" get different sorts of responses from people. I've found "do you think you could explain how to do this to someone else" to get more productive and cheerful responses, unless I'm talking to someone who already thinks of themselves as a meetup organizer. My working theory is that the implication that you expect someone to run this meetup spooks people, but that's basically wild speculation. Ideally I'd like to put the idea in their head that they can in fact run meetups and to get them to think of themselves as potential meetup organizers.
The ultimate goal of this is to make each of your attendees into people capable of running the meetup themselves. Done successfully and over a long timespan, it makes for an abundance of potential meetup organizers and a community that's more resistant to individuals becoming too busy or burning out or moving to the Bay area.
Late arrivals are perfect practice targets. Sometimes your meetup started at 1, you explained things at 1:15, and then just as you're getting started someone new shows up. This can be frustrating, but it can also be a wonderful opportunity to get someone else to do the explanation! This is especially helpful for activities with short "rounds" where someone can easily jump in, and invaluable for large activities where you may not immediately notice when someone new shows up.
This one isn't at a quick trick, but it's a good trick: Write down how to run your meetups and incorporate it into handouts. Petrov Day is an amazing example of this. When you attend Petrov Day, you often get handed a script. If you take that script with you, you can make copies of it at home and then run your own Petrov Day next year basically by just following the script. Now that is a self-documenting meetup.