Can they be diagnosed by a general practitioner in a physical ?
The General probably can refer you to a blood testing center and they can handle it from there. So yes they can diagonose you with an std based on the blood test. Or for females go to the OB/GYN he can diagnose you with simply a pap smear. some symtoms include
-stink discharge
-colored discharged such as yellow, grey, green
-abdominal pain
-some std cause you to have yeast infections, so u might be itchy. just depends