How do you tell whether your muscle aches are from dehydration or something else?

Try rehydration. If you prove you are not dehydrated, then the ache relates to something else e.g. Cholesterol meds or exertion. Usually people do not get dehydrated without an obvious reason. Examples include nausea, vomiting, profuse sweating, diarrhea, or water pills all with inadequate intake of liquids to make up for the losses. Soup, broth are a good way for repletion of fluids due to salt and water content.