Are you seeing/feeling the babies move around at all? They usually get pretty active within the last couple of days ... Momma will begin to look around for a place to deliver so be on the watch for leaving closet or other doors open - those places make wonderful birthing nests but aren't very convenient for those of us in charge of care for the kittens. Momma may vomit or be observed straining in the litter box a day or so before delivery.
Not being able to actually feel her belly, I'd say you are going to have a large litter on your hands, I wouldn't be surprised if she delivers at least 5 babies.
For your own peace of mind, might give your vet a call in the morning if Momma hasn't begun to show signs of labor.
Best of luck and let us know how it goes,