The hiv test looks for antibodies in a person's blood. When hiv (which is a virus) enters a person's body, special chemicals are produced. These are called antibodies. Antibodies are the body's response to an infection. So if a person has antibodies to hiv in their blood, it means they have been infected with hiv (an exception is the case of an hiv negative baby born to a positive mother, who will retain her antibodies for some months).
Most people develop these antibodies within 3 months of infection. In rare cases, it can take up to 6 months. It would be extremely uncommon to take longer than 6 months for antibodies to develop.
Getting tested earlier than 3 months may result in an unclear test result, as an infected person may not yet have developed antibodies to hiv. The time between infection and the development of antibodies is called the window period. In the window period people infected with hiv have no antibodies in their blood that can be detected by an hiv test. However, the person may already have high levels of hiv in their blood, sexual fluids or breast milk. Hiv can be passed on to another person during the window period even though an hiv test will not show that you are infected with hiv. So it is best to wait for at least three months after the last time you were at risk before taking the test. Some test centres may recommend testing again at 6 months, just to be extra sure.
It is also important that you are not at further risk of getting infected with hiv during this time period. The test is only accurate if there are no other exposures between the time of possible exposure to hiv and testing.
The only way to know for sure whether you are infected with hiv is to have an hiv antibody test. It is not possible to tell from any symptoms.