It's a sensitive subject, but I say ask.  Being Maid of Honor isn't some achievement of a lifetime, but it is important.  There are things that the Maid of Honor normally helps take care of (bridal shower, bachelorette parties, random errands, etc.)  You don't want her thinking you are her Maid of Honor, but you're not planning certain things because you're not sure.  And just say that. "I need to know so I can get started.  If not, that's ok, but I want to be clear."  I know, I had a similar problem when my husband proposed.  One of my friends thought she was going to be MOH and when I didn't say that, she asked me.  I answered, crisis averted, and she still danced at my wedding.