The girl might miss her period. But when she does a pregnancy test the result can either be negative or positive. When she tests positive she has no indication the pregnancy is ectopic.
The first signal is usually a (one sided) stomach ache and possibly some blood loss, which is a bit brownish.
The pain and blood loss increase as the pregnancy progresses. Other signs are cramps, shoulder pain, a false urge to urinate, pain with a bowel movement, sweating, feeling cold, dizziness and fainting.
If you suspect an ectopic pregnancy seek help right away. The embryo needs to be removed as soon as possible. Placing it in the womb has never been done successfully.