Without the help of mill markings to identify a given steel beam, you should measure the height (A), the flange width (B), the flange thickness (C) and the web thickness (D). With that information you can go to any number of steel service center catalogues or engineering guides to match the measurements with the beam name. The American Institute of Steel Construction (AISC) provides a comprehensive list.
For example, if a beam has a height of 15.72in, a flange width of 15.80in, a flange thickness of 1.560in, and a web thickness of 0.980in, then it is a W14 x 211. The “W” indicates that it is a wide flange beam (as opposed to, say, an S beam); the “14” is referenced as section number; and the “211” indicates that the beam weighs 211 lbs per foot. You should note that the permissible variations from the specified dimensions can range from 1/8in to 5/16 depending on the beam size.
And if you want to know the metric equivalent of any beam size, check out our new Metric Conversion Calculator. It calculates that a W14 x 211 is the equivalent of a W355.6 (mm) x 314.39 (Kg/m) whose section number is W360 x 314.