Ok. I found a site that said this, "With the approach of ovulation it becomes softer, opens, and raises its position relative to the uterus. At infertile times it is low, closed and firm." But this opening, at the peak of your fertile period, is only 15 mm wide. We are not talking very big so you're not likely to notice it. What you are likely to notice is that your cervix is LOW and SOFT. From what I can summarize, and I'm not a health proffessional just a mommy whose been through two births, if you were pregnant and it was period time, your cervix should still be pretty high (difficult to reach) and firm to touch.
I know that when I was looking for signs of impending labor with my second, I kept checking for the softness of my cervix. When my cervix stopped feeling like an unripe peach and started feeling like a mushy banana, we were getting somewhere. If you want to be pregnant, you'll be looking for your cervix to be high and hard.
As to the opening you are looking for, it's probably going to be too small for you to notice with just your fingers. I would notice the feel of your cervix and how high it is. For your period, the cervix should be soft and low.