They are starting to figure out the biomarkers for longcovid, so if you can see a doctor who isn't dismissing longcovid as an attitude problem they might be able to run tests. My coworker had bloodwork done and the doctor said it was long covid, one of them was a low white blood cell count, and the other was related to blood sugar/insulin response, showing elevated blood sugar despite them not having anything to eat for half a day. It's been a year since their infection, which happened just over a year after their last vaccination.

If your heart starts racing out of nowhere, your blood pressure swings wildly up and down, or you get winded or have to rest for a long time after doing simple physically active things that you had no problem doing before a covid infection, that's usually a sign. This fucking virus can infect every part of the body, so it's probably going to be hard to diagnose all the issues it can cause.