Never intervene internally until well after you see the water bag. If you get the water bag then nothing for several hours, then you might gently investigate.
Once labour has started (the water bag has been ejected), if she isn't sufficiently dilated and the lambs are trying to come, then yes you can help her open up by massaging the cervix. But if you are a novice I really wouldn't be getting into that - if you think you may need to do this, get an experienced person to have a look and tell you whether you just need to be more patient.
As everyone else has said, we too have had ewes this year seemingly well over their times, the size of houses, bags like cows' and fit to burst - but they eventually do their thing when they are good and ready. And yes, that usually means when some foul weather comes.