I vent all my now-it-all-ism on bikeforums.

Seriously, when I see someone with apparently big problem, I offer advice politely if it's a good opportunity. That is, they seem open for it and not in a rush, or deep in their thoughts. People mostly react well.

When I see someone pushing a bike with a flat tyre, I'm even rude enough to ask where they live. If they're over 1 km away, I offer to fix their flat - done that about 5 times so far. If they're close to home, I just ask if they're OK and if they need any help, if they don't ask for a patch kit, I don't offer it. Never had anything but positive reactions.