From what I've always read, a fertile cervix can often be described by the acronym SHOW - soft, high, open, wet.
Soft- it should feel like your lips, where firm feels like your nose
High- it should be far back in your vagina. Some women cannot feel their own cervix when it is high.
Open- It shouldn't be anything gaping, just a very very small opening. A couple months of tracking and open should contrast with closed by feel.
Wet- A fertile cervix is often surrounded by EWCM- egg white cervical mucous that is clear and stretchy, just like egg whites.