How do contractions feel?

Hi ladies!

So, I am 39 weeks pregnant AND I cant sleep due to some weird pain I have in my lower back that comes across my pelvic area. Its a stabing pain and goes away for some time and comes back. Its not painful as in not being able go breath.. its just there and causes pain. If I move my legs up, it hurts, down, it hurts so there is no way I can be laying down without being uncomfortable?

What signs am I looking for in order to know I am starting labor?

or that they are contractions?

Thank you!!