Fillers may be possible, but the only way to tell would be to watch her smile and see how the skin around her chin moves when she does that. (That means keep watch next time she's on TV!) If Madonna has a chin implant, it won't move when she makes facial expressions.
Personally I think Madonna doing any type of chin augmentation is a bad idea. First of all I don't remember her chin being weak to begin with. Secondly, on women, an over prominent chin isn't what would be considered an attractive physical trait to have .