Please tell me how soon can you tell you are pregnant?

Possibly: Get a home pregnancy test. They should turn positive about the time you're a week late. If it's negative, repeat it and call your doctor.

Day 35: Your pregnancy test can be positive even within 2 weeks of conception. However, it can take more than 3 weeks to be positive sometimes. It should certainly be postiive by day 35 if you have regular 28 day cycles.