Would you behave differently if you knew that you were HIV Postitive?
While we know that some types of sexual activities are much lower risk than others (with some activities only being a theoretically risk, not a proven risk) and the proven effectiveness of condoms, there are no guarantees. The fact is, it is very difficult to be 100% certain that you did not engage in any risk behavior, especially if you have any sort of unprotected sexual activity with a person known to be HIV positive or you do not know what his or her HIV status is. Abstinence—or sexually activity only in a monogamous relationship in which both partners are HIV negative—is the only way to eliminate the risk of HIV infection.
The only way to know for sure whether you have been infected, even after an activity that is considered to be very low risk for HIV transmission, is to get tested!