How do you know if you are having an ectopic pregnancy?

Hi i was seen in the ER yesterday because of sharp pains like baby contractions i don't remember when my last period was, so they could not tell me much. They think maybe i had appendix problems but could not rule that i had it or didn't. I am really worried they said they could no see the sac in the sonogram so i could be early early on or that i could have an ectopic pregnancy. the cramping has stopped and i have not had any bleeding or any other symptoms. Please if someone could give in great detail how i would know, sn: I took the pregnancy test last wednesday so.... Thank you for your help