One of the tricks you learn in NLP is to spot which modality someone is accessing by observing their face while they think. I find it a little simplistic. Is it likely that everyone looks horizontally to the left when they are accessing auditory memory? No. And yet when I tested it, by asking someone what they had been listening to in the car on the way to work, her eyes flicked horizontally to the left before she answered me.
Another idea is that the words you use betray your modality: 'I see what you mean', 'I can get hold of that', 'I hear you'. Too simplistic? But could it be true?
Another one is whether there is an association between modality and, for want of a better word, personality. For instance I think that (although I am pretty clumsy in the real world) I am kinaesthetic in my thinking. That is, I think in physical sensory metaphors (ideas feel tight, or they slip away etc.) That is associated with faults like being careless and hasty.
Visual people are supposed to able to grasp multiple relations between things, while auditory are supposed to be more linear, but also more accurate and flexible (or something). And there is supposed to be another type, which is predominantly verbal and conceptual.
Do you have a primary mental 'modality'?
Mixed (say more in comments)
Don't believe in it