Gestational diabetes

I'm 26 weeks and haven't had to do the glucose test yet but I just had a few questions.

1. If I eat a lot of sweet does that mean I'll have it ?

2. Will I be able to tell if I have it by my babies weight in ultrasounds? At 24 weeks my little man was 1 pound 7 ounces, like would he weigh a lot if I had it? Or it that just towards the end?

3. At what point do you have to take the test? Also, I've had blood work a few times would that be able to tell if I had it ?