Experience mostly. After triggering OI problems or crashes dozens of times after different activities (or non-activities), it's pretty clear which symptoms are coming after which activities. But this is a problem that probably affects most or all patients new to having ME/CFS. OI symptoms can be especially vague and difficult to ascribe to a particular cause - hence education about OI and PEM causes, symptoms, and avoidance is critical. Studies would be nice of course, especially for educating doctors. But there's not much biomedical funding for ME/CFS as it is, and most researchers are focused on finding answers that will make a big difference - treatments or cures, or at least mechanisms.