While standard steel bolts are marked by a graded system of head markings that identify strength, stainless steel bolts, often chosen for their resistance to rust and corrosion, are not held to a system. Instead, they must be identified in other ways.
Although not as strong as hardened steel, stainless steel bolts are constructed of low-carbon steel alloys, 17 percent to 19 percent chromium and 8 percent to 13 percent nickel according to BoltDepotcom; they are chosen for their resistance to corrosion compared to other bolts.
The most easily verifiable physical difference between a stainless steel bolt and a standard steel bolt is through magnetism. Stainless steel bolts are far less magnetic than their steel counterparts and they may carry a more noticeable luster.
While there are no industrywide standards for stainless steel bolt head markings, some common markings are giveaways. The absence of standard steel radial line marking is an important indicator, but abbreviations using some variation of B8 are common among stainless steel bolt manufacturers.
If you know the name of your bolt's manufacturer, you can obtain a chart from the manufacturer to exactly match your bolt's head markings with its construction material. The American Iron and Steel Institute (AISI) designations of 304, 316, 409, 410 and 430 denote stainless steel.