It's all RNG unfortunately. I personally used the 15 days method (skip ahead 15 days from current day, check for thought bubble, skip 15 days if thought bubble not on desired villager, rinse and repeat whilst avoiding Saturdays and event days) but I think there might be better methods out there as this method involves such large amounts of time travel. One thing to keep in mind is that the most recently moved in villager will never have a thought bubble.