Determining "consentablity" in behavioral health clients in forensic episodes

One of the challenges I have experienced as a forensic nurse, is determining whether a client in the midst of a behavioral health episode, can provide consent for a forensic exam. Is it ethically reasonable for FNEs to expect an individual, involuntarily hospitalized for behavioral health evaluation, to be able to give consent for a forensic exam? If so, what is the instrument do we use to assist to determine capability to consent? Does this instrument exists? Would an standardized instrument be helpful in obtaining consent in these circumstance? Any observations or comments would be appreciated and would form the basis of a research question.