So...a volcano in Japan caught us all napping. Vulcanologists had no idea that Mount Ontake was going to erupt - and while a small eruption, it caught a lot of hikers on the mountain, with at least one person killed and a number trapped in a mountain lodge.
Vulcanologists are always looking for better ways to predict eruptions, including imaging systems - the latest being tested can look up to 20 miles under the surface to follow magma movement. Most of the US systems are tested on Mt. St. Helen's, which last erupted 10 years ago. (St. Helen's is definitely recharging, but whether she'll erupt in years or decades is still up in the air).
In the mean time, if you go hiking on a volcano...well...be careful and aware that you're taking a risk. Sometimes we can predict eruptions.
Sometimes, as happened with Ontake, we can't.