When I have a target bird to venture after I like to study up in my field guides and look at online photos before I go. Going through the exercise of putting some slides together emphasizing the unique and comparative characteristics helps me ingrain those traits in my brain.
The plates below show both breeding and nonbreeding plumage for the Loons possible to see in Idaho. Most of the comparisons I make are to the Common Loon. My focus was mostly on nonbreeding plumage as that is what I am most likely to see here during the winter. It is looking like that neck pattern will be the single most important characteristic to look for through the spotting scope. Then the bill perhaps.
(Thanks to Terry Gray, Corey Finger, and everybody else's photos I borrowed from Google Image Search)