The most common way is based on size. The technique is known as size exclusion chromatography, or gel permeation chromatography. The sample is passed through a column that has a porous solid material. The smaller polymers can work their way into those pores, while larger molecules don’t interact with the solid material. The biggest molecules, because they don’t interact with the solid phase, come out of the column first followed by smaller and smaller molecules. The time it takes for a polymer to get through the porous column is related to its molecular weight (okay, technically it’s based on the hydrodynamic volume, but let’s let this approximation slide). In practice, these instruments are calibrated using standard polymer samples of a known molecular weight.