Delineate Property Line with Neighbor (WA State)

[Manco Snead]

Greetings,

I am hoping someone can give me a bit of feedback; I own a duplex in Washington state and the new owner of the duplex next to mine wants to build a fence separating our properties, which I am fine with. Both lots are rectangle in shape. However, despite all the documents I have for my property, along with assessor/auditor internet resources, I am having a difficult time finding a map which accurately depicts my property line. The area where the property line is roughly located seems fairly obvious, but I want to know more precisely its location. I live outside the duplex location area and am very rarely able to make the trip to deal with issues such as this.

Any ideas of how to approach this?  Much thanks.