You just have to do it awhile and you learn what is low
vs high (ie knuckle deep to reach it
vs full middle finger
vs full ring finger). Then soft vs hard would be the difference between your lips and nose. I know way to much information, but you asked;-) It is also best when you cervix is wet (ie egg white stretchy discharge). Just remember most fertile is SHOW--Soft, High, Open, Wet. Again sorry I can't help with open vs closed. Good luck.