Easy,

Find a magnet, put it on the pipe, did it fall off? If so its stainless

If it stuck its not stainless and can rust as the Chrome content in relation to the Iron content is not enough to make it a stainless steel which must by definition contain 18% Cr to qualify as a stainless steel.


Ordinary stainless steels as used in exhaust systems are nearly always 304 and sometimes 316L are both non magnetic stainless

Al.