It depends on the Ovulation, really. Your period happens exactly 14 days from the day you ovulated. If your period is generally regular, you can figure out the exact week of ovulation and avoid sexual intercourse during that week (Called the "Rhythm Method", though it is not 100 percent.) Pregnancy tests are most accurate at at LEAST 5 days after your missed period.
By 14 days after fertilization (about the time you would expect your period), some home pregnancy tests may be able to detect the hormone in your urine and give you a positive result. But in spite of their claims, most are not sensitive enough to guarantee you an accurate result at this point. In fact, you're much more likely to get an accurate result if you wait until a week after your expected period before testing.