It's really a gradient, because physicians and health care workers in general do capacity assessments all the time. When you're giving someone medications or performing surgery you have to ask permission first.
Where it gets complex is when you have people with mental health issues or cognitive issues, which may affect it. Sometimes, even though I've been doing these now for 30 years, for some people I cannot say one way or the other, so the default setting is that they are capable unless they can be deemed incapable.