Not sure if this is what you are looking for, but I use the LabelPointReplacer to get the ParallelRotation direction of a line.

• GeometryExtractor (FME Binary) to backup the geometry to an attribute.
• LabelPointReplacer with label 1 and labelheight 1 to replace the line with a label.
• AttributeExposer to expose ParallelRotation.
• GeometryReplacer to restore the original geometry from the attribute.
•  

Once you have the directions of the lines in attributes, you can calculate if it is 90 degrees different from the one you are testing.

In addition to @nielsgerrits​ very novel solution, I can think of two more approaches:

• There's a HorizontalAngleCalculator custom transformer available on the FME Hub, which calculates the angles of line features, a line with both x coordinates the same will have an angle of 90 or 270 degrees
• The CoordinateExtractor can extract coordinate values into a list (_indices{}), assuming you have a 2-point line you can then test to see if _indices{0}.x = _indices{1}.x

Thanks @nielsgerrits​  and @Hans van der Maarel​  both solutions worked perfectly :)