Lots and lots of practice. It's still highly subjective, and two different experienced practitioners can check the same cervix at the same time, and get two different answers.
It's also never exactly accurate, because every woman's cervical thickness is at a different starting place. The average starting thickness is around 3-4cm of thickness, but again, it varies quite a bit from woman to woman.
For me, I pretty much only give a handful of answers when I try to ascertain effacement: thick, 25%, 50% 75%, 90% and 100%.
What's less important than the number is simply if it's getting thinner. Which is why it's always best, if possible, when trying to determine if a woman is in labor, to have the same person check her. Because I can tell if she feels thinner than the last time I checked her, but my 50% might be someone else's 75%.