I do not know that whether Magnesium Sulfate can be detected on postmortem examination. but I know that Magnesium Sulfate make all never and muscle relaxed, painless and easy to death (euthanasia)....... However, you may compare with and without the higher Magnesium Sulfate' postmortem muscle tissue sections to see whether there are different, or look for the method which can be detected the concentration of Magnesium Sulfate on the postmortem muscle tissues......