There are different methods for identifying the grade of stainless steel, depending on the level of accuracy and detail you need. Here are some of the most common ones:

Using a magnet: The easiest way to identify the grade of stainless steel is to use a magnet. Ferritic and martensitic grades will stick to a magnet, while austenitic grades will not. However, this method is not very precise, as some austenitic grades may have a slight magnetic response due to cold working or welding.
Using a stainless steel identifying reagent: The best way to correctly distinguish stainless steel grades is to use a stainless steel identifying reagent, which is a chemical solution that reacts with the metal and changes color according to its composition. This method is more accurate and reliable than using a magnet, but it requires some safety precautions and equipment. You can buy stainless steel identifying reagents online or from specialized stores.
Using the SAE or ASTM grading system: The most comprehensive way to identify the grade of stainless steel is to use the SAE or ASTM grading system, which are standardized numbering systems that classify the metal based on its alloy composition and crystalline structure. The SAE system uses a three-digit number that starts with a 2, a 3, or a 4, while the ASTM system uses a six-digit number that starts with an A followed by a series of numbers and letters. You can find the SAE or ASTM grade of stainless steel by looking at its label, certificate, or specification sheet, or by using online databases or reference charts.