"Electric Comet" wrote in message
news

making an outdoor piece and wondered what the black cabinet screws
are coated with and if they are suitable for outdoors

black {iron} oxide
Yes as long as you don't mind rust

but i wonder if there are any simple tricks to test for different
coatings that anyone has

maybe a vinegar test would do

Just use bronze - not brass - or stainless steel. Galvanized is OK too but
hot dipped galvanized fasteners have become scarce.