You can test with a 4th generation antibody/antigen at 28 days. Results will be very accurate. Any other test would be conclusive at 90 days from your exposure.
If you had unprotected vaginal sex, then you should test at 28 days with the 4th gen duo as Noriko already told you. It would be wise to speak to your doctor and make sure you do this. Remember, a one time exposure is low risk even if she is HIV+. But you do need to test and if it has been a month now, you can do it this week.