Have to test for erectile dysfunction? Wouldnt you just know?

ED testing. Testing for ed (interview, exam, nocturnal penile tumescence test, bloodwork, etc) is used to evaluate how severe ed is & to determine cause. Treatment is most effective when it addresses underlying cause, not just symptom. Ed can be caused by stress, poor circulation, hormone problem, nerve problem, psychological factors or combination, & different patterns of erections suggest different causes.
Yes. How to test for ed? Try to get an erection. If you can't and it bothers you, then you have ed. So yes, you'd know right then & there. It's what we refer to as a clinical diagnosis. There's no blood test per se to make the diagnosis. However, we might draw blood to look for a potential cause of ed. But we should also ask about relationship, stress, health, lifestyle habits, etc.