One simple approach is hand people old objective tests for subjects like math under the original rules. If people where simply becoming more competent in all areas then average scores should be higher today, but that’s not what happens.

Measuring complexity more specifically is probably easiest done by using equivalent jobs and compare how long each take to achieve competence. McDonald’s cooks take less time to train now vs the 1970’s, but I am not sure how that compares across industries.