If you have had babies before your cervix will be 1-2 cms open at this stage as a normal thing.
effacement happens with dilation in a multi ( someone who has given birth before) but in a primip ( first baby) it usually happens before dilation. So if its not your 1st baby, you wouldn't expect your cervix to be effacing until you are in labour - I'm presuming you have given birth before, forgive me if I am wrong

As TT40 says, assessing effacement is something that you learn after doing a LOT of vaginal examinations and is quite subjective to a degree, i guess if you are a habitual examiner you will pick up the difference between your cervix length pre and during pregnancy.

I would be scrubbing under my nails with a nail brush too prior to examining!