The cervix looks like a tube, between 3 and 5cms in length, with one end at the top of the vagina (the external os), and the other end inside the womb (the internal os).
In normal labors, cervical effacement can occur weeks ahead or not until labor begins. Judging effacement begins with measuring your (or your partner's) middle finger and finding where 4cm is located.
To check effacement, find the cervical os (opening), then run your finger up the outer portion to the upper vaginal wall and estimate the distance there. To make the whole process even more complicated, we don’t measure how many centimeters the distance is, but the percentage that is gone, ie. 50% effaced, 80% effaced, etc.
When the cervix is 100% effaced, then it is unable to be felt at all... called "paper thin" because the cervix is flush with the baby's head... barely a perception of change between vaginal wall and baby's head... a fraction of an elevation... like when you put your hand on a sheet of paper lying on your desk and slide it off with your eyes closed... like that.
If the cervix is 1-2cm long, that's about 50% effaced. Less than 1cm is 75% or more. 100% is not usually until 10cm dilation. However, women can have 100% effacement pre-labor. 100% effacement is a fairly reliable predictor of a quick or even precipitous labor.