Verified answer

I remember seeing this thing blow...rather I remember seeing the glacial outburst flood it caused (you should youtube it, it's beautiful and awesome). The only way volcanic eruptions can be predicted is by earthquakes or tremors (caused by moving magma), perhaps the caldera domes up and drops dramatically, or more steam is seen exiting the volcano in its caldera or out its flanks.

I'm not sure what the signs were for Eyjafjallajökull, but these are some tell tale signs that let scientists know she's gonna blow. Once they get these signs, they can better predict when the event is going to happen, but not with 100% accuracy.

not really

Nobody knew anything was gonna happen until a few months before the eruptions, when a series of earthquakes occurred beneath the volcano. Only then did people know it was going to erupt soon