How did scientists know that the volcano Eyjafjallajokull as going to erupt?

Well, honey, those scientists aren't mind readers, but they do have fancy gadgets like seismometers and gas sensors that can detect changes in the volcano's activity. So when they saw an increase in seismic activity and gas emissions, they put two and two together and predicted that Eyjafjallajokull was about to blow its top. It's all about observation and analysis, not crystal balls and magic spells.