Originally Posted by mstateglfr

A magnet will tell you if something is aluminum or steel. it wont stick to aluminum.
The bike would have come with steel everything. If its all original- its a terrible deal as itll be heavy, dull, and expensive to overhaul and/or update.
If updating has been done thru the years and those steel parts have been replaced with al alloy, its still an expensive option but it is at least closer to worth the asking price.

To specifically answer your question- look for aluminum rims, seatpost, stem, and handlebars. That will make a lighter bike and will typically mean its a bike from higher up in a brand's line.
Alloy chainrings(the things up front connected to the crank and pedals) are also a general sign of a higher end bike since they usually came on mid-range and up bikes.