I don't have any really clever (e.g. scientific) answers Ken but here's a few of my (very) simplistic solutions…
a) If it's rusty, it's probably not stainless (but isn't necessarily silver steel either of course)
b) I only buy stainless stock that IS non-magnetic – I have a little magnetic plastic 'knob' to test with…so it's a very good test of my own stock…
c) If it's 13" long (and maybe even a little rusty) then it's quite probably silver steel
d) If I'm really not sure if it's silver steel (and I need it t be) – a small sample heated red-hot and dunked in water/oil should be dead hard afterwards.
e) Some of my silver steel is stamped "Stubs" (where you have that 13" 'end' cut from the metre length) – and I try to cut from the other end of these bars (when I remember to do so)…retaining the stamp on the left over stock for future reference…
So generally variations on being magnetic (or not), rusty (or not) or able to harden (or not).
Generally one of them will work for me. Hope this helps.
Regards,
IanT