SS 304 magnet or not?

http://en.wikipedia.org/wiki/Stainless_steel

From:

All austenitic grades have very low magnetic permeabilities and hence show almost no response to a magnet when in the annealed condition; the situation is, however, far less clear when these steels have been cold worked by wire drawing, rolling or even centreless grinding, shot blasting or heavy polishing. After substantial cold working Grade 304 may exhibit quite strong response to a magnet, whereas Grades 310 and 316 will in most instances still be almost totally non-responsive.

Deformation-induced martensite.

This is a simple question to answer. Get a big magnet - the kind that rips your fingers off when you hold it near a chunk of iron - and touch it to a piece of 304. Then touch it to a 304 screw. Repeat as many times as you need to get the idea.