It's easily explained here. Www.rune-tips.net. Great community full of us merchers.
How they predict is:
1. Usually items have "waves." This is when the item falls and lowers in a wave.
2. Items are being dumped by a major manipulation clan
3. The item is overpriced and is way above it's average line in the GE graphs.
If you're looking to get into merchanting I recommend joining www.Rune-tips.net and reading the investing guide. THIS IS NOT A MANIPULATION CLAN. You will learn how to solo invest.
Using graphs and knowing price trends for different items helps you when predicting price changes. Or if the items price is being manipulated, joining masser public chats and checking the items and dump dates/prices will help.
You can't predict it. They watch the increased rate by %. The grand exchange shows how high or low a price has risen within the past day, week, and month. If it increases tremendously, merchers take advantage and manipulate the price.