How do you know if you're dilated?

Natalie • Happily married to my best friend. Expecting our 2nd child.
I'm 37 weeks + 3 days. I have seen a lot of posts by women around as far along as I am mentioning they are this dilated, etc.
How do you know? Is it because you go to the hospital and check or your doctor checks you at your appointments?
My midwife sees me every 2 weeks and has never checked me to tell me if I'm dilated..