I'm wondering at what point you think "asking" becomes "nagging." Does it have to do with how many times you ask, or more with how you ask? Or does it depend it more on what it is you are asking your SO to do or not do? For example, if there was something important that needed to be done by him and you could not do yourself, and he was continually not getting it done, is it nagging if you remind him several times regardless of how you do the reminding? I can't think of a specific example right now as it is not a current issue, but I would like to keep the thought in mind so I can audit my own behavior.

When I think of nagging I am usually envisioning the tone of voice and body language used. That's really a more extreme example like you might find on a sitcom, and I think there's a large difference between overt and subtle nagging.