How long ago were her other pregnancies? Did she have glucose tolerance tests with those pregnancies? If so is there a way to find out what her numbers were?
I had my first child 18 years ago and my BG was high enough that had they used the same standards that they do now, I would have been diagnosed with gestational diabetes. But at the time the standards were much different. (Thankfully they have changed things, and many more women get help.) So even if they did test with her previous pregnancies and didn't think she was diabetic, it's quite possible that her numbers were high enough to qualify if they were using todays standards.
I think you're right to be concerned, and it would be a very good idea to keep an eye on her fasting sugars. It would also be a good idea to ask if they are willing to do an early glucose tolerance test on her when she sees her midwife.
At the time of my last pregnancy the recommended fasting sugars for gestational diabetes were below 115, but my endo disagreed. He said that since the baby should be using up some of the sugar that was normally in my blood, I should be aiming for slightly lower than 'acceptable' results. I worked hard to keep my fasting BG under 100 and keep my after meal BG under 135. Alicia weighed 3kg when she was born, so I believe he may have had the right idea.
Don't stress yourself out too much over things though. You know about this disease, and how to manage it if she starts showing signs of high fasting BG. You don't have to stress her out about it either, just ask if she would be willing to humor you and poke her finger every once in awhile? For gestational diabetes they are not just concerned with fasting BG though, they are also worried about how well the body recovers from eating sugar, so every once in awhile checking her after meal BG might be a good idea.
Relax if you can, and trust that if she shows signs of gestational you'll be able to help her get through it. I think it's wonderful that she'll have your knowledge and support to help her through things if she is diagnosed.
And congratulations on being a grandmother again

!