Because DMSO has a freezing point of 68 F, one is able to tell its approximate concentration, if a bottle of the liquid solvent is acquired from an unknown source. Put the unopened bottle into the refrigerator (not the freezer). Within two hours the liquid will turn solid, like ice. You now have 99.5 percent DMSO, the purest and highest concentration made [as of 1993]. Leaving the bottle cap on will prevent hydrolyzation (decomposition) so that the liquid will freeze ate 68F or less.
If, when the frozen bottle is turned upside down, little rivulets of water flow through the ice, you probably possess the veterinary grade DMSO. This is a 90 percent concentration. Ten percent is distilled water.
If the DMSO doesn’t freeze while standing in the refrigerator, put it into the freezer compartment. If it does not turn solid even after standing at below 32F, it probably indicates a 50 percent mix of DMSO and water. If it does turn solid in the freezer, this liquid is not DMSO, or is almost all water with just a small bit of the solvent mixed in. Fifty percent DMSO is an antifreeze; it will work well in automobiles for winter driving. DMSO, Natures Healer, Dr Morton Walker, pg 32-33.
This testing procedure is supported by the Dimethyl Sulfoxide (DMSO) Physical Properties, November 2005, Graph 9 on page 6 showing the freezing temperature/density of DMSO with water, Gaylord Chemicals. It shows that at between 90 percent to 100 percent density DMSO will freeze at temperatures between 32F and 65.3F. It also shows that it will freeze between 0 percent and 37 percent density between 32F and -13F. The lower temperatures freeze (in the freezer compartment) the water in the lower densities only up to 37 percent density. Densities between 37 percent and 90 percent will not freeze in the standard home refrigerator/freezer.
For more information please refer to this article for more tests