The gestation is around six weeks, if I remember rightly.
When is she due?
In the last week they tend to sleep more than normal, also depending on the set up you have in the bedding area she should make a nest normally close to giving birth. The nest will be just like what she normally makes but with a fur lining that she will pull out.
Ive had ferrets that you could hardly tell that they were pregnant, only time will tell for you. If your still not sure I would make sure she is on her own and has plenty of bedding material (I use straw), and left to her own devices.
Hope this helps a bit