What part of the "pacific northwest"? I'm in central coastal CA, and I "feel your pain". Although there were toe-holds of Spanish influence here by the 1770s, (permanent settlements, verses simply exploration), the entire non-indian population of CA, up to ....... say .... 1800, was numbered in the very low 1000s. And since the indians had no refined metal, there is simply nothing to find before this era. Oregon and Washington had some dabbling and exploration in the late 1700s as well. But realistically, it wasn't till the 1820s or later, that you had any outposts or permanent occupation, by non-indian metal carrying folks. We are/were on the "remotest parts of the earth" at that time. Heck, they still thought CA was an island, on some maps, to as late as 1800 in some maps
As for the rules of other countries, I'm sure you'll get answers, website links, etc.... Just be aware:
Whatever answer you may get from the *technical* angle. It may be couched in terms of national antiquities type stuff, ship-wreck salvor and export type laws, and other such federal level stuff. For example, if you were to ask the same question of some lawyers or bureaucrats in the USA: "Is it legal to detect in the USA?", you may get a "no", because they're answering in terns of ARPA, or Mel Fisher legal hassles, or federal level parks and antiquities type stuff, etc... But the reality is, detecting goes on here with no problems, as long as you're not being some sort of nuisance, raiding pyramaids, sneaking around Shiloh or Bodie, etc....
An example is Mexico: Over the years, people have posed the same question about Mexico. They ask a consulate or whatever, and get a "no", so they leave their detectors at home. Imagine their surprise, when they get to their tourist beaches in Cancun, Cozomel, etc... and see that detectors on the beaches are a common site. Doh! Again: a technical "treasure" and "riches" and "antiquities" answer, verses a casual hobbyist loose-change answer.
You might also check to see if there are dealers for wherever you're going. Like Whites, Garrett, Fisher, etc... They have dealers in most foreign countries. If they list any for your country, then you gotta figure that they can't be illegal, lest who would be buying them?