How do you tell a difference between implantation bleeding and a period?

Wait and test. A positive pregnancy test can tell a woman that prior spotting was actually implantation spotting (assume implantation does not cause notable bleeding). It can be hard to tell if a woman is pregnant because some of the symptoms seen in early pregnancy can also happen in non-pregnant women. A urine pregnancy test is very reliable by 3 weeks after sex (sex approximates conception).