How do I know if I'm in labor?

I have been having cramping all morning on and off but I'm not sure if they are contractions or Braxton hicks. The cramping is coming pretty often. I'm not sure how it feels when labor starts. With my daughter I had no Idea I was in labor with her. I was having cramping and went to my Drs office, they hooked me up to a contraction monitor and told me what I was feeling was contractions & then when they checked to see if I was dilating I was already 5cm. I had my daughter a few hrs later.