How to tell if pregnant on multiple month birth control?

Im on a multiple month birth control pill, i dont have a period every month, only every three. So the convenience of having a period once a month to alert me im not pregnant dosent happen. I just started my birth control a week ago, and me and my boyfriend had protected sex last night, and without a test, i wont know if im pregnant for three more months. If there's really no way to tell ill go out and buy a test, but like i said it was protected but pregnancy is very scary for me. Can anyone help because ive been looking at different forums but haven't found an answer?