Hard to tell from just weight gain. One thing you want to look for is the glow. Most females will start to get much lighter with blushing and overall coloration turning much brighter. I have females that look like fires when they glow, others the head blushing gets real light and faded.
The ovulation is the most important thing to see as that tells you exactly where you are. You should see the ovulation then 2 weeks later she will shed and then 30 days after that she should lay. These are approx times.
The glow leads up to the ovy so if they glow, they go!