How do you know when you dilated?

i asked this ? an didn't get any answers really but can you do it at home i know people who say they did it my mid wife checked me last month an she said my crevices were thinning an please if your gunna say please wait for your next doc appt. its dangerous ...... all that please explain how bc to me i dont see how it could hurt me an my baby when my mid wife checks me she sticks her finger in an that's it she can tell so how is it dangerous ??? really im just asking what do u feel for how do you tell the diff