In almost all these cases this fume is just steam from rain or snow that has seeped into the volcano far enough to encounter got rocks.
Experts are able to tell a lot by the color of the fume.
For some volcanoes any of these plumes do indicate impending activity. If even only a steam plume is noticed on a volcano that has never had such a plume before, then perhaps there is magma moving upwards within the volcano and you need to worry about it. It could turn out that maybe there was just an extra-heavy rainstorm or snowfall, however. As you can see, much of volcanic prediction relies on recognizing patterns and comparing them to past patterns. It is very important to realize that what may be an important sign at one volcano might not mean anything in particular at another.