my experience checking my own cervix was really interesting, particularly looking back on it:
in pregnancy, including up until due date: could check my own cervix standing in the shower pretty easily, even as it became somewhat (but not very) posterior and as my belly got bigger and bigger.
in labor: i checked once in the shower, and things felt so foreign down there (as things move down and tighten and such). i guessed that i was around 5cm based on the new contours inside, but it wasn't like a hole, cause the baby's head was right there... it was just, well, contours.
the midwife came about two hours later and checked me. i was around 7cm with a small lip, so i think my guess when i checked myself was right, despite my inexperience.
then labor continued but the lip swelled more which kept my cervix around 7 or 8 for the next 5 or 6 hours. i never checked it myself during that time... we decided to do AROM near the end of that period with the hope of bringing down the babies head more evenly since it was presenting kind of funny, possibly contributing to the swollen lip. still have mixed feelings on that, but moving on... midwife did one more check after AROM. i was around 8 and strechier, but still not major progress or anything.
i got back in the tub. my body started pushing. being my first baby, i didn't know what things would feel like. i felt something starting to come out, but i didn't know what it was (baby, duh)! i tried to check myself but everything was hard and i couldn't find anything (again, baby was right there, duh)! in retrospect, it now makes sense. the final progress was just really fast and i spent the whole time trying not to push and the baby just came out so fast.
here's the catch: shortly after checking myself and being too nonverbal to say anything even along the lines of "something's there," there was an unusual incident. i was in the birth tub and apparently (i didn't see it but everyone else did) a rather large plume of blood shot out of me during a contraction. i could tell that something was up from the tone my midwife used telling me i had to get out of the tub now so she could check me (gentle and kind but COMPLETELY authoritative), but i still wasn't worried or anything.
when i finally hauled myself (with much help from everyone) to the arm chair next to the birth tub, the midwife was at the table getting some equipment when the doula said "the baby is coming!" and then seeing my confused look, she smiled at me, "your having a baby!" (i wasn't completely sure the rest of me wasn't coming out too, until she said that). next contraction (with me trying to slow things down and not push) out came the nead. next one after that, the rest. our baby!!!
so, what was up with the sudden out-of-tub neccessity? because the last check had shown me incompletely dialated, and i had JUST started having push contractions (the baby came down within 15 minutes, my first baby) she was worried i had torn my cervix and that had caused the blood. in fact, i had just torn my periurethral area a bit which bleeds a lot but is not a major complication.
so, the moral of the story is that i found checking myself to be really useful and informative, BUT i didn't have the familiarity with what a birthing cervix felt like nor did i have the verbal ability to communicate what i was feeling to our birth team which would have been quite useful for me to relieve the concern about the blood and to be able to stay in the water, which i wanted to do.
i feel much more confident about doing my own cervical exams next time, now that i have an idea what things will feel like down there. of course, for a first birth, without access to anyone elses cervix for practice, i'm not sure how you gain that experience, other than expecting things to feel different than your used to.