Short term, they should follow their sugars and American College of Obstetrics and Gynecology, and the American Diabetes Association recommend that they get tested for diabetes about 6-12 weeks postpartum with a 2 Hour OGTT. If seen more than 3 months after delivery, it's reasonable to get the ADA recommended A1C since it's easier to get and doesn't require 2 hours of a new mother's time. The fasting sugar is also recommended, but compared to the OGTT it can miss 85% of people with impaired glucose tolerance. Finally, this all ties in with their long term risk of getting overt DM.

It is a real issue as up to 5% of women will have diabetes diagnosed immediately after pregnancy.