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.