How about a quick dab with a magnet to give a clue as to composition?

If you have got cast iron but the bottoms are blackened with lighter shades up the inside of the pans there is a fair chance they are stove enameled. Staining of this kind is pretty much impossible to remove, and most chemical methods will take the gloss off the enamel, making sticking/burning more likely in future use.

The good news is that such a stain, though unattractive, has no significant effect on the performance of the pans, and won't taint your food. Le Creuset advise use a low heat, and never to dump the hot pan into cold water. They also offer a stain remover, I have not tried it, but have heard only lukewarm reports of its performance. Under no circumstances take metal utensils or scourers to an enameled pan, and scouring powder it also a no-no.

Some cast aluminium pans are pretty hefty, you might try taking steel wool to the underside, if it comes up shiny under the burning this might be what you have, and you could proceed to the insides in relative confidence.