You mean by scraping it and seeing if it rusts, or something else?

Generally, the easiest way is with a magnet. The most corrosion-resistant of the common stainless steels are austenitic. These steels are not ferromagnetic, or very weakly ferromagnetic if they have been cold worked. Cheaper and less corrosion-resistant ferritic stainless steels are strongly ferromagnetic.

Martensitic stainless steels are less corrosion resistant than austenitic steels and are ferromagnetic, but they're used for good reason. Unlike other types, martensitic steels are hardenable through heat treatment, making them the only suitable choice for many sharp-edged tools or parts subject to severe abrasion. Stainless steel cutlery is most commonly an 18/0 ferritic grade, but premium cutlery may be an 18/10 austenitic. Kitchen knives are invariably martensitic, because ferritic or austenitic grades wouldn't hold a sharp edge.