I don't usually "hook" a cervix, either. This is why - if I can reach enough to be
able to hook it, I can generally feel enough to estimate dilation based on the amount of curve that I
can feel, even if I can't feel all the way around the circle. (Make sense? I.E. if you feel half or a third of a regular soup can rim vs. a rim of a little bottle of vanilla you are going to know the difference). I have done it, but I just personally don't usually find it necessary.
Also, I find that sometimes just asking pts to lie on their hand/fists isn't enough for them to understand what I'm getting at/why.. I make sure to tell them that by doing that it gets their bottom up higher and it usually makes the exam easier (
everyone is usually on board with that plan, right?!!) - then they tend to gladly get that bottom way up in the air, and get a better pelvic tilt going (get that lower back in a "c" shape with pelvis tilted upward) than simply sitting on their hands and not making much positional change.
I also usually do like SBE said, run my finger all the way up the posterior wall, then up and anterior until I find it.
When you have a low baby head there like you described, sometimes you can do the opposite and run up around that head as a guide until your finger steps down on the lip of the cervix since it's
got to be there continuous with the baby head somewhere!
I'm still fairly new but I've come a long way with those exams

That said, I still get people to check behind me any time I'm not sure - I'm not comfortable guestimating; even though some things are generally going to be the case (the primip high and posterior or whatever) - as sure as I'd do it, it would be the
one person that didn't fit the bill & would go have a baby on the side of the road somewhere! LOL