How do Ontario courts decide what standard of care a professional owed me?

Ontario courts measure a professional against what a reasonably competent member of that same profession would have done in the same circumstances, using the knowledge and practice standards that existed at the time - not with hindsight, and not against an ideal or perfect practitioner. A professional isn't negligent just because a different, equally competent professional might have made a different judgment call, or because the outcome turned out badly. The question is whether the choice made was one a reasonably skilled member of the profession could reasonably have made.

Because this standard depends on specialized knowledge courts don't have on their own, expert evidence from someone in the same field is almost always required to establish what accepted practice actually was and how the professional's conduct compared to it. Courts also look at things like professional guidelines, common industry practice, and any applicable regulatory standards, though following common practice isn't an automatic defence if the practice itself was unreasonably risky. Different professions and specializations can have meaningfully different standards, so a general practitioner and a specialist, for example, are not judged against the same benchmark.