Notebook fan stuck on?

I'm not sure when this began, but I noticed recently that, once my
notebook's fan gets turned on due to the temperature rising, it seems
never to get turned off.  I'm not sure how to begin debugging this -- in
particular, how to decide if it's because the notebook isn't cooling
properly (perhaps due to ACPI or hardware?) or if we're failing to turn it
back down against once it is cooled (perhaps also due to ACPI?). 
Regardless, it actually has the effect of generating quite a bit more
noise, and using moderately more battery, so any pointers on where to
start looking for helpful.

Thanks,

Robert N M Watson