STI's (STD's) It depends on the sti's (sexually transmitted infections) you are referring to. Painful or painless rash. Burning on urination. Penile discharge. Some sti's can be fairly asymptomatic and thus testing is your best bet to know if you have one (or more than one).
Silent assassin. If you have unprotected sexual intercourse with someone that you are not 100% sure does not have a sexually transmitted infection (sti), then you need to be tested for all common sti's regardless of your symptoms. Hiv may be a silent assassin.