I don't know about on AIM; but on MSN it will ask if you will allow them to see you. You can choose yes or no. That is all that really matters as if you don't let them see you they don't know whether you're on or not?
I don't think there's a way to see if they have you on though?
I know that on aim, u can select to have only people on your buddy list be able to contact you. But to my knowledge it is not possible to see if people have you on there buddy list in aim.