If you know where to start, near the intersection of Sprague and Arthur (a.k.a. the sharp curve at the west end of Erie Street Yard), you can follow it southeastward from there on most good maps, especially USGS 7.5-minute topos. In some places it's shown as an abandoned RR, in other places just a faint dashed or dotted line, in other places there's just an obvious linear gap between local streets which were dead-ended or curved to meet the edge of the RR right of way.

The end papers of Walt Grande's "The Northwest's Own Railway" (SP&S) and the map on page 193 give a pretty clear glimpse, too. In the latter, the GN branch is shown as the Spokane, Coeur d'Alene & Palouse Ry, which is one of the many names this once-electrified line had before it was acquired by GN.