The property boundaries are not always visible, but they are defined in government records (in the US, typically with the local property tax assessor’s office). It’s not uncommon for a lot boundary to be temporarily marked with stakes and twine during a construction project such as building a house on an empty lot.

Ultimately, if an object like a tree is on your property, even if it’s right up to the line, that tree is yours (but your neighbor also has the right to prune any branches that extend onto their property). Your neighbor could take down the laundry pole if they wanted to.