When fitting a hearing aid for an open fit, how do we know when it is and isn't?

References:

Mueller, H. G., Ricketts, T.A., 2006 "Open-canal fittings" The Hearing Journal: November - Volume 59 - Issue 11

Winkler, A., et al. 2016 “Open Versus Closed Hearing-Aid Fittings: A Literature Review of Both Fitting Approaches.” Trends in hearing vol. 20.