Some people can get infected with an std and not show symptoms, but be able to pass it to others. The best thing to do is get a blood test or females get a pap smear as some std's are discovered this way.
std.about.com/od/overviewofstds/a…

Since there are several STD's out there, there is no way of specifically knowing which one you have through symptoms. you may never show symptoms but still pass them through sexual behavior. Tests' are the only way of knowing….unless you have a green or yellow discharge through your anus then your infected.

IT REALLLY DEPENNDS ON THE STD SOMETIMES WITHIN 24HOURS -7DAYS

Many people still wait for the definite proof or the symptoms to confirm that they are really STD infected, far from their knowledge that symptoms do not appear immediately after their exposure in a certain infection.With regular STD Testing in your Local STD Testing Centers, you can keep track of your reproductive health. Preventive measures are also taught by concerned health specialist.