That is pretty much the only thing I hate about living in the city. It's hard to figure out when offers of help are genuine or not. It does really happen that someone offers to "help", and then demand a fee once they are done. For example, one time our car got stuck on a side street. A bunch of guys ran over, and offered to unstick us.
Once we were moving again, they demanded money.
So, it is hard to tell, and while its an obvious joke in much of the country, if you are in a big city, not so much.