I've never seen a pre-departure boarding pass check unless there was a specific problem like someone already in someone else's seat, but they usually have a seating chart with names for taking meal orders. If someone is in a seat that should be vacant, they often ask to see a boarding pass

Switched within the same cabin to be next to someone? Fine. What would you like to eat?

Ought to be in a different cabin? Please return to your assigned seat, sir/ma'am.