How will they know if it's ectopic?

So on Saturday I found out I am pregnant, I don't know how far due to irregular periods but got negatives the week before so must be very early. I was getting mild cramps so went to the early pregnancy unit on Monday. They did a vaginal ultrasound but she couldn't find a baby or a sac or anything. She also said she saw a mass on my ovary which could either be a cyst or an ectopic pregnancy. I had blood tests on Monday (my hcg was 133) and I had another one this morning. I get the result tomorrow. I am curious as to what they are looking for in the bloods today? How will they know if it's ectopic or not? Will it be 100% certain? Also, I have done 8 tests now, and the line is getting darker every day, does this mean anything?

Thanks