Well having long fingers like mine helps. I can find the -3 w/o problem. H ere is how I do it:
Have pt's head as low as possible.
Have pt place her hands beneath her buttocks, to elevate her pelvis--you can also place a bedpan with a towel or chux over it, to pad it, under her buttocks, for even more elevation. Works like a charm.
Ask her to put her feet together, knees apart, to open up the pelvis even more.
Check along the small of her back (posterior wall of vagina) and keep going all the way up-----then move anteriorly, and viola, you will find a that high, posterior cervix that so eludes many of us. If it is closed/thick, you will feel dimpling, as you probably well know. Effacement is not that all-important. You can tell if it's thinning. As you know, the uneffaced cervix is about 1 inch in length. So, go from there......estimate it is all you can do. I can never get over people who can call a 25 or 30% effacement, for example.
You REALLY can tell that cervix is 1/3 of an inch, even when closed? WOW ......I can't. I can usually just go with 0/50/60/80/90/100. Beyond that, it's all so objective anyhow.
Don't get all worked up. It DEFINATELY takes a lot of practice to get em all----and after 9 years at it, I still have people check behind me, whenever I am unsure. NO ONE expects you to be the expert. Don't be afraid to ask for backup in any case where you are unsure, including assessing labor progress.
Hope these tricks help. They never fail me.