Does the tree have to stay or can it be removed?

Roughly 8 feet from the sewer line, there is a row of 5 hemlock trees that,
according to my measurements, are right on the property line between my
property and my neighbor's property. In addition to this, there is a large
maple tree a few feet behind one of the hemlock trees, which is completely
on my neighbor's property.

I assume I would need my neighbor's permission to remove the hemlock trees,
since they are exactly on the property line, and those trees were there when
I moved into the house so I don't know who originally planted them. I'd
also hate to remove the hemlock trees and then later find out the maple tree
is the cause, since the hemlock trees provide some degree of privacy.

Jeff