Is there a way to tell if user filtered on year-month vs year in year-month hierarchy?

Hello. 

I have a Year-Month hierarchy [Date].[Year-Month] and several YTD and Parallelperiod Calculated Members using the heirarchy.  I need to change the calculations to do a different calculation (IIF block) based on whether the user chose [Year] ie 2012 or [Year-Month] ie 2012-5.  Is the month NULL when only year is chosen?  What condition can I use for this?  I spent sometime looking online and trying code but can't find anything that works.

I would appreciate any input.