You don't. You have to wait. I was 9 weeks when we found out we were having twins and that was by accident. I was having sharp pains, passing out and couldn't walk. My ob had me rushed to the er and they did an emergency u/s to make sure the baby was okay...turns out there were two and one of my guys was laying on my sciatic nerve which was causing the sharp pains and not being able to walk. If I hadn't gone I wouldn't have found out until I was closer to 20 weeks since my ob doesn't see pregnant patients until the 2nd trimester.
there is no way to tell that early, most women don't even know they are pregnant at this point yet. you will have to wait til at least 6 weeks pregnancy and get an ultrasound done to see.