I check while sitting on the toilet. This may be tmi, but since you all are asking, sometimes I have to use my other hand to push my fingers in just a bit farther to reach my cervix. I just use 2 fingers (washed clean before hand of course). Make sure you have enough lubrication so it's easy to get up in there.
As for how to find your cervix, surely you've noticed how uncomfortable it is when it is bumped from baby or during sex, or during an exam. You should be able to tell when you find your cervix because it's has more nerves than the rest of the vagina and is a bit uncomfortable when you touch it. Make sense? If this is not your first baby, it will probably be at least slightly open already (like 1 cm) if not more at this point. If it's more, it's kind of like a tunnel unless it's really effaced and open. HTH!