To me the best evaluation of an admissions process would be the success of students once through the system. I am not sure what would otherwise be evaluated? Perhaps that can be expanded upon in the question? In any case, I am sure many departments run tabs on where alumni go and I can only share the statistics from two departments my Alma Mater and my current workplace.
Figure 1. Statistics on post-graduate careers of students from the Geology department of University of Minnesota
Figure 2. Statistics on post-graduate careers of PhD students from the Physical Geography and Quaternary Geology department at Stockholm University
So from this perspective the admissions system is evaluated from the perspective of employability within the field.
To study the process itself one would need to set up some additional criteria against which to evaluate the process but I am not sure what those criteria would necessarily be. Some aspects such as gender and ethnicity bias are of course identified but it is unclear if there is a need to try to actively change the interest of applicants other than strive to reach under-represented groups. Such outreach activities are, however, not part of the admission process.