May I ask how to know if we have a clean wife void or not (newbie) I don't want to visit someone else's town and them having unwanted vilagers because of a trade / visit.
You have a clean void if you haven't let any villagers go without someone adopting them first.
I don't know if you know this (if you do, skip ahead to the next paragraph), but when a villager is in boxes, you can let a player who has an open spot in their town go to your town and talk to that villager. They'll get an option to invite that villager into their town. If you do this, the villager will simply go to them.
If nobody has adopted a villager who has moved out, they'll end up in your void, so they'll move in randomly into the towns of people you've streetpassed or wifi'd.
The best way to keep your void clean is by letting people adopt your villagers, you can sell them or give them away here on the forum (or elsewhere, for example, tumblr blogs dedicated to trading villagers).
If your void isn't clean, you clean it by visiting the town of someone who already has 10 villagers. The game will try to make your voided villager move into their town but since they already have a full town, the villager will simply be deleted from your void.
I hope that helps.
By the way, you can switch off streetpass by going to the Mii Plaza (I think that's what it's called) on your 3DS main menu. After you get through the initial steps, there's a tool box there. Select it and you'll be given several options, including switching off streetpass. You can also check under your 3DS settings menu if everything you want switched off is switched off, I think.
As to wifi, there isn't any way to check if someone's void is clean. You just have to trust the people you communicate with to have a clean void and let them know if that's a rule for communicating with you via wifi. Lots of giveaways, for example, specifically state that you must have a clean void to trade with them so if you're posting a trade or visiting people, you can do something like that and ask if their void is empty. If you're on this forum and make a giveaway/trade/sales post and have that as a rule, if someone lies about it and you get their unwanted villager, you can give them a bad wi-fi rating, I think (but I'm not sure).
I hope that helps! Let me know if you have any other questions.