HE can be diagnosed by your Dr. (Generally a GI Dr. Or a Hepatologist). They will go by your symptoms and can also do a blood test to check your ammonia level. Blood tests are not 100% accurate all the time. I have had a normal ammonia level and still had symptoms. It's a disease of symptoms, not numbers.
If you ever feel "off", there is nothing wrong with asking your Dr. to check your ammonia level.