How can I tell the difference between aluminium, steel and galvanised steel?

I don't see what difference it makes. If it is already painted, just lightly sand and prime and then paint it. Unless you are going to sand it all the way down to bare metal.

Galvanising is a zinc coating with a big crystal structure. It looks sort of cyrstally and snowfalaky with patches the size of fingernail. Aluminium is gnerally pale and not magentic... Steel is magnetic galvanised or not Also lighter then steel. Not sure how else you tell

If a magnet will stick to the door it is steel, if it is painted then it is doubtful it is galvanized.

Aluminum is non ferrous so a magnet will not stick to it, and it is softer than steel. After you have determined that, if it has old paint on it, you might want to use a primer that corresponds to the paint you will be using either vinyl or oil based, then paint. It is probably not Galvanized, but if it is, the unpainted surface with have a two color metal Spackle look.

a magnet

Use a magnet will not pick up aluminium

To answer your question without all the details, is a magnet will stick to steel, and very, very slightly to stainless steel so slightly that you can barely notice it, aluminum will not attract a magnet.

it wont be ally it would more than likely to be steel is it the frame or door..doors are not normally metal

Aluminium is light and oxidises white. Steel is heavy and rusts, galvanised steel is crystaline in appearance and the galvanisation tends to peel at the edges.