They can approach you and begin a conversation, but you do not have to acknowledge them if you so wish. If someone called on you and you do not appear to be doing anything illegal, they can not detain you. You can ask if your being detained, and if they say no, walk off. If they say yes, tell them you wish to know why, that you will remain silent, and would like a lawyer. You can ask what statute you have violated, which you would know you haven't violated one, but I wouldn't say much more. If it ever came to this point, you could take them to court after the fact. I know of a supreme court ruling that stated you could resist unlawful arrest, however I can not recall the case, and I would not resist. You have no obligation to show I.D., or identify yourself. The only time you must produce your drivers license is when you are driving.