To test for nosema you will need about 40 bees. The test should be done in spring and you should collect the older flying bees.
A good way of doing this is to make a tube out of a waxed cardboard drinks carton. Cut one end off at an angle and then place a clear plastic bag over the other end. It helps if you can seal the bag easily. When your bees are flying, place the open end over the entrance and wait. Bees leaving the hive will fly up the tube and collect in the bag. When you have around 40, remove and quickly seal the bag. To kill the bees humanely, place the bag in a freezer overnight. The bees are now ready for testing. Only the abdomens are needed, so they can be cut off with some sharp scissors.