what will you do when you see someone crying on the road.

I think that it is very hard to say what is bothering you to a stranger. You did the right thing, you acknowledged her pain, it was her choice whether she wanted to talk about it or not. I am sure that you touched her life with her concern even if you weren't able to solve her problem. It's amazing the impact that a kind word or smile can have on someones day.

I will Just watch her/him . . . ^_^ . . or maybe try to ask..

It would depend on the circumstances... I did see a lady crying in the car park a few months back so I just said," Excuse me, are you alright? Can I do anything to help?" She wiped her eyes, smiled and said she'd be fine but thanked me for asking. I think there are people who would become aggresive and tell you to mind your own business though. I would guess that the girl was a little embarrassed that you'd seen her crying in public. Most people wouldn't want to reveal too much to a total stranger in the street. I think the fact that you cared enough to acknowledge her sadness was probably enough.