Personally, regardless of whether or not they look like they have it handled, I always ask them if they would like any help. If they are really bothered by my asking, then that's on them. I usually get a simple "No Thanks. Got it Handled.", and I keep moving on.

If I see that they are in distress, obviously frustrated, or walking their bike, I will stop first, approach, then ask if I can be of assistance.

There was one woman last year that met all 3 of these criteria (saddle was nearly vertical) - and she declined my offer at first, but I insisted - showed her that I had the tools to help. After all was said and done, she was very thankful.

Usually it's simply polite to ask - sometimes it's best to take the initiative, and be persistent. Some people are shy and don't want to 'bother' anyone with their troubles.

But that's just my opinion...