Hcg levels

How do people know their hcg levels? I’ve never had a problem with fertility and my first daughter I had a good pregnancy, got pregnant both try on both now I’m due with my second in feb. I’m 5 weeks along. My ob scheduled me in 3 weeks when I’m 8 weeks along but how do people know their hcg levels? My lines are dark and getting progressive but why do others worry or are told this? I have never heard of it before joining this group