http://www.guineapigcages.com/forum/...arts-info.html is a great link to a chart with veggies/fruits on it and how much you can feed and how often. It also tells you how much calcium, vitamin C, etc, is in each one.
Wow 600 grams. My foster piggie gave birth 3 1/2 months ago and the girls from her litter are over 600 grams already. The boy from the same litter was 850 grams a week ago. It depends how they were fed though. You are taking a piggie from poor conditions who is undernourished, so of course they will be tiny. My grown piggies are all in the 1100-1200 gram range. It varies so widely. The biggest thing is to keep track of any weight gain or loss. I'm sure you will get weight gain as you give them good food, but if you see a big, continuous weight gain, then she's likely pregnant.
There really is no way to tell other than watching and waiting. Unless you want to pay big money for a vet to do an ultrasound. I'm not sure how far along in a pregnancy a sow would need to be before an xray would show the babies for sure - you could look into that.