Also N apis tends to peak through the winter (likely because of bees with dysentery being confined within the hive) and can 'disappear' through the summer, whereas N ceranae seems more like a year-round problem.
It isn't easy to tell the difference under the microscope. Should you want confirmation, the NBU will do a proper test (for a fee).
Careful when mentioning dysentery and nosema in the same sentence. Neither of the Nosema's causes dysentery, but the latter certainly exaccerbates the spread of the disease. Depends which examiner you have looking at your paper as to how they mark this question despite guidance!
Apis has been historically known as 'Spring dwindling' and certainly colonies that fail to build up in the Spring are frequently found to have a bad dose of Nosema apis. Also, N. ceranae does not like the cold and builds up in the bees much more in the summer (see research on nosema in Spanish colonies). Whilst pooing outside in the summer seems to help the bees to get rind of apis spores, the heat makes ceranae more virulent. Therefore, a colony that goes down hill when apparently all else is fine (flow good, weather OK etc) i the summer, might well be found to have N. ceranae. Left to its own devices the colony may collapse completely leaving a few remaining bees to beg a home in nearby hives thus continuing the spread of the disease.
There is a National Honey Show lecture video on Nosema due up soon on their web site:
Nosema - a pervasive and persistent parasite of the honey bee - Robert Paxton
DNA is the only 100% way to tell. Some comparisons, and it's not either but both are frequently found if you get down to actual analysis. Also a lot about how N ceranae developed, spread and appears limited by climate. Some interesting stuff about potential interactions with pesticides IIRC. Possibly a case of a question that could be answered differently now to what you'd expect in 2009.