side to sude / up down noise question

I've heard of acoustic engineers who check this.... insulation, etc.

Simple. If the building has an external fire escape, avoid at all costs. Fire escape=shitty construction.

If it's a new development, I would come with a friend and have them go next door and upstairs. Talk loud, walk around. That is the best way.

I resales, it is much more difficult. Any building built in the 50's - 70's, you will hear everything, unless it is a High Rise. In a High Rise, you will have good insulation up/down, but not side to side.

Knowing the layout of apartments next to you, helps as well. Some developers placed bedroom to bedroom and living room to living room, this way you are almost certain to have some peace when you are asleep.