Hi Rich
A competent person is someone who can discuss and understand the design loads and the capability of the structure to support the loads. The designer is one person and the designer is somebody. Before the erection of such structures, there are stability calculations that need to be carried. Other principles that forms the basis of scaffold design include strength and rigidity of structure,the intended use and application of the scaffold; the safety of persons engaged in the erection, alteration and dismantling of the scaffold; the safety of persons using the scaffold; and the safety of persons in the vicinity of the scaffold. Scaffolders may be experienced and skilled but remember that one must be competent by skill, experience and qualification.
Now coming back to your question, is there any training/qualification required? I would definitely say YES, designers must be engineers.
I'm pretty sure others will add on this, but I'm open to discussion.
Hope you got an idea of what is expected from a competent designer.

thanks

Vrick