Experts agree that one HIV test is not enough. A series of tests is required to assure that you are indeed HIV negative. It is recommended that you be tested at the time of exposure, 6 weeks later, 3 months after exposure and finally some even believe that a test at 6 months post exposure is necessary.
Note that if another potential exposure occurs during the series of tests, the series must start over.
If you are HIV negative after the 6 month test you can consider yourself not HIV infected.