First Time Mom Question

I have been having period like cramps with some mild back pain since I woke up today. I called my obgyn’s office they told me “To keep an eye on it and to come to L&D; if I think it is labor.” I said okay and was about to ask how to know. Well the nurse hung up before I could ask. I’m a first time mom, so how will I know if is time to go to L&D;?