Having a good friend who is handicapped (blind), you sometimes need to approach this very carefully. Many people with disabilities don't want help (and I have been shot down trying to offer assistance). Sometimes the best way is to do it in an off-hand manner. Example: At a bus stop, don't asked a person if they need help finding their bus. Rather casually ask which bus they are riding, then say something like, "Here comes the 23 bus".