As I think you know from your text, this is going to be subjective.
As you note, the level of liability is reduced to reasonable skill and care i.e. the process and knowledge applied, so ask the architect to demonstrate the process followed by reference to notes etc.. If he/she just knocked up a sketch with no thought, then he/she would probably have not used reasonable skill and care. If he/she referenced and research standards, spoke to the end user, had the end design checked etc. the situation may be different.
As you note, the level of liability is reduced to reasonable skill and care i.e. the process and knowledge applied, so ask the architect to demonstrate the process followed by reference to notes etc.. If he/she just knocked up a sketch with no thought, then he/she would probably have not used reasonable skill and care. If he/she referenced and research standards, spoke to the end user, had the end design checked etc. the situation may be different.