Unless the vet does an x-ray or ultrasound I wouldn't totally depend on his/her word whether she's pregnant. We made that mistake with our first doe and were totally unprepared because we believed the vet.
As far as behavior, you can't always depend on this to know if they're pregnant. Not all does eat more and not all does get aggressive. They can even fool you by nesting and then not having anything. By day 30-32, though, you'll know.