You can't really hide anything on a Mini. If all that is beneath the rust is rust and not filler then you will know it is a genuine car that just has the usual Mini rust problems.
Its when you see one that is wobbly and wavy with a nice fresh coat of paint that i would be worrying.
If you are worried about major stuff thats been covered up with filler, take along a magnet but if it looks original then it's a good start. Its better to chop out a crusty bit of metal and replace it than to have to grind out inch thick filler from a dodgy repair.
If you are interested in it, perhaps also take along someone who knows this stuff to get a better idea.
You will be very hard pushed to find a Mini with no rust on it, unless it has been properly restored and looked after since.
Ben