Almost always not magnetic but impossible to identify without analysis, magnetic chrome iron is frequently mistaken for stainless steel but is worth only a fraction of the price. 5×2 is the accepted size anything larger is oversize, material should be free of other metals or contamination.
Similar to stainless steel in appearance but magnetic and can rust slightly. Cheap sinks are made of chrome iron, can be expensive bought new but low value compared with stainless steel. 5×2 is the size otherwise it is oversize.