You would notice discharge if you had it. Burning with urination can be a symptom of an STD, but this doesn't always happen. Any type of discharge, with or without burning, should be checked out - it could be an STD.
Em
So, discharge would be very noiticeable. How about the burning? Discharge with no burning indicates maybe some other UTI?
Any discharge from the penis should be taken seriously. Of course, symptoms can vary from mild to severe, but gonorrhea usually produces pretty noticeable symptoms in men. I wouldn't say it's especially easy to distinguish an STD from some other infection. NGU can be caused by many different bacteria and is not necessarily an STD (even though it is often caused by chlamydia). Bottom line - testing is the only way to be sure. It's important to get treatment if you have an STD, or any other infection for that matter.
Best wishes,
Em