my dd2 was born at 36w2d after 5 days of regular, painless contractions. i literally went to 8cm before i had pain with contractions, so i would agree with the pp that it's frequency that you need to be aware of.
on the other hand, i have had lots and lots of episodes with all of my other pregnancies where i had regular contractions for hours with zero change to my cervix.
the big difference with dd2 was that i was also having a lot of other labor signs along with the contractions....crampiness low in the front that wrapped around to my back, aching thighs, diarrhea, etc.
that probably does nothing to ease your mind, i'm sorry. there is little way to know unless you have a cervical check. just rest, drink water, and watch your body for other signs of preterm labor! good luck, mama!