How to know I'm pregnant for 7 weeks?

First you confirm pregnancy. Then you talk to your doctor. They'll give you a guess based on your last period. You'll have a better idea about how far along you are when an ultrasound is done, but even then they won't be able to tell you exactly. It can be off as much as 2 weeks in either direction.

If you're asking this because of paternity, please know that there's only one accurate way to determine paternity if you had sex with two or more different men around the time you conceived, and that's a DNA test after the baby is born.

Good luck.

Are you asking if you are pregnant or a specific time pregnant,

If you are pregnant then you will have missed your last period. Have you?

As for how many weeks, that is just estimates and there is no way to tell exactly