Other than using the Protractor tool to draw a 90 degree guide, is there another simple way to do this? I don’t see any inferences when for instance I attempt to draw one line perpendicular to another. For a Bezier curve which I want to extrude a rectangle along, I’m going to draw a guide using the Protractor tool normal to the end of the curve first (drawing the initial base reference line for the protractor parallel to a segment of the curve), then use this as a guide for the rectangle.
In much more recent versions of SketchUp there are inferencing tools to help with that. But not in that ancient SU8 that you are using. You could use Perpendicular Face Tools to stick a face on the end of the curve and use it as a reference. You can get it from the Extension Warehouse.