You have the Appelbaum and Grisso criteria, which are well known and internationally recognized, to guide the competence assessment. They are really cognitive, so a patient has to be able to make a decision, explain their decision and use all of the needed information and apply it to themselves.
There are supplements in the Netherlands in our MAID guidelines with an assessment of the emotional congruence of the decision. Especially in the context of eating disorders, we are asked to see if there aren't any pathological values involved in the decision to choose death. In short, that's how we do that. There are pretty extensive, comprehensive guidelines for how to do that.