Most items made from stainless steel are made from 304 alloy or equivalent. 304 will corrode a bit, but usually the corrosion can be buffed out. The commonly available stainless steel, that is the most corrosion resistant is 316 alloy. It is not common to find 316, except for the marine, food processing, etc, industries.
The magnet test works most of the time, but that is not always the case with 316 that has been cold worked. Cold working, may make 316 magnetic.
Another source of corrosion in 304 and 316 is dirty tools, tools that have been used on carbon steel or iron, sparks or particles from nearby machining of carbon steel, etc. Carbon steel or cast iron particles will corrode stainless. If left too long before cleaning, they will compromise the chromium oxide surface on the stainless and will result in localized corrosion. Many suppliers do not understand this. They do not have tooling dedicated to stainless only, or they fabricate parts from other metals in the vicinity of stainless.