I don't believe you'll find anything in the rule book about it, since the only distinguishable metals in the stories are aluminum and a few of its alloys. These have no lines at all, being "Allomantically inert". Other than this, no primary viewpoint character ever makes a distinction, and the Inquisitor viewpoints aren't detailed or common enough to have hinted if their superior ability in this field allowed such a distinction.
Things to try that have canonical support:
Lines to smaller metals inside larger, hollow metals (sub-lines are less notable at first glance).
Vary object sizes, or have shaped symbols under an obvious surface.
Thin rods inside hollow cylinders inside the obvious surface object (Push the internal rod while Pulling on the cylinder of the set that is third from the left, and Pull the internal rod of the second set from the right, and Push the cylinder of the leftmost set, while not disturbing any of the others. Do that in order, holding all steps in position, in order to set the first dial of the external twist-knob combination lock. New patterns are required for each external input, plus a final pattern to allow the unlatching mechanism to release. Most of the moving parts are stone, wood, or aluminum to hide the mechanism design from the intruder.)