Getting back to the OP's question. Most reasonably recent department store bikes will probably have stamped steel brakes, and either Shimano's cheapest derailleurs or derailleur brands you have never heard of. Saddles may have a steel base pan. Also look at the frame welds as a lot look very bad. Rear dropouts will be stamped steel or aluminum. Cheap one piece cranksets may also be used. If aluminum 3 piece cranksets are used then look at the chain rings which may be not replaceable and are all steel.

With a little practice you can learn to spot the lowest end bikes pretty fast. In fact stop by Walmart and Target and take a look at their bikes to get an idea of what not to buy in the used market.