Steel vs Aluminum Spokes and Rims
You want stainless spokes and aluminum rims. As suggested in the comments you can check the rims with a magnet – and also by appearance steel rims will probably be shiny. But steel rims are kind of rare, most likely you'll have aluminum on any reasonably priced bike.

For the spokes it may be hard to tell with a magnet. I think the easiest way to tell would be to look closely. Stainless spokes will have no finish and will be "good looking" – no, or minimal rust (sometimes steel from machining will leave some residue that will rust, but it will be superficial, color but no significant oxidation). Steel spokes will have some kind of surface finish to prevent rust. You can probably see the finish and some rust or minor damage to it. Finally look at the heads of the spokes. Name brand spokes like Wheelsmith, DT, or Sapim will be marked. Wheelsmith uses a 'W', I think DT uses 'DT', and I'm not sure about Sapim. If you see one of those marks it is a decent chance that the spokes are stainless – as long as they aren't the dull gray of a galvanized (zinc coated) spoke.