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.
OK, there is a lot of hearsay ... Points to consider
1) There is more than one type of steel. Steels with varying elasticity and tensile strength, yield strength, failure limits, hardness, toughness can be made. The same is true for aluminium.
2) Steel was bad as a wheel material because caliper braking systems in wet weather would not work properly. Aluminium wheel rims did not lose friction so much in these conditions.
3) Aluminium wheels are popular, light, easy to manufacture and obtain. They are probably more developed than steel counterparts because there is a market.
4) Elasticity does not mean weakness. Why would structural engineers build skyscrapers from steel girders? A material with high elasticity can bend or deform to a different shape under extreme stress and still return to its original shape. (Aluminium is classically less elastic than steel and fails earlier under extreme stress).
I'm not sure about the actual properties of the aluminium used, but I'd be surprised if it was more rigid than the most rigid steel. (more like a coke can)