It is fairly easy to test for nosema. All you need is a microscope giving
400x magnification and some bees.  You will probably find some.
Nosema is endemic (at least in the UK) but usually only becomes a problem
when the colony has some other stress as well.  Try finding and eliminating
other stress factors and moving the colony onto new comb or old comb that has
been fumigated with 80 per cent acetic acid as soon as you can.
Fumagillin on its own won't work as the bees will become re-infected as soon
as they clean up old comb with the nosema spores on it.
Chris Slade