If you click the spoiler "Other similar methods that have been suggested" there are a few different methods.
Also, if you are friends with the villager they will ping faster.
EDIT: ALSO! If you cannot find the pinger, talk to other villagers until they tell you who is thinking of leaving. If they are not outside, save and quit then reload and find them. If you are not on speaking terms with them, talk to them once, save quit then find them again.
Going a day at a time and talking to the villagers all around will help until someone pings you or someone says a villager is moving.
From there either get them to ping and ask them to stay.... or just TT 5 days and they will be in boxes if you want them to move.