I think even if you just have a really hard impact but dont see any cracks, its still recommended to buy a new one.
100%, read any helmet manual and it'll say that after any impact it needs to be changed.
That said I bonk my head quite often against trees, sometimes the snow, rocks. I haven't changed helmets yet because I haven't seen any damage yet to any part of the helmet. Even all those little bonks are making me think it's time to get a new one again soon...