AFAIK there's no easy way of doing it through the API. You would need to iterate on the elements to check which attributes they are using (which is a very convoluted proccess given that different element types have different set of attributes - including the nested element types of complex structures, such as the different components of railings, stairs and curtain walls) and then find out the attributes that didn't have any hit.
I've done something similar in scope with a different goal (finding elements with missing attributes) and I can attest that this is a very convoluted and error prone endeavour.
