you can test yourself, by taking epsom salts baths.

6 to 8 oz of epsom salts to lukewarm bath tub and soak at least 1/2 hr before bedtime, for several days will show you benefits if your body needs this mineral.

There are many clinical causes of accelerated magnesium loss.
Diabetes (both type I and II) causes the kidney to excrete more magnesium daily. Heavy use of diuretics including coffee (caffeine), use of some drugs (like prednisone) and many others will put you on a depleted path.

14 Drugs That Deplete Magnesium by Suzy Cohen - Jigsaw Health

The addition to this list of the acid blocking drugs, is fairly new and use of these drugs is very common now. Hence magnesium loss can be severe in people who don't know this.

If you choose to take a magnesium supplement orally, your target dose should be 1/2 of the RDA. that is 200mg elemental. Check the labeling for ELEMENTAL doses, since not all types are labeled this way. Elemental measures only the magnesium portion and not the total weight of the product. Example... 1000mg of magnesium malate has 150mg of magnesium per tablet, and the rest is the malate portion. So one of this type per day would be your goal.

I prefer SlowMag supplement, as it is easy on the GI tract (little if any diarrhea) and one of these twice a day is adequate for most people. It has a generic called Mag64. SlowMag is in a delayed matrix and releases slowly. I myself use a topical lotion product and find that better for myself, as my GI tract is very sensitive to oral types.
This product was formulated originally for doctors to use for their cardiac patients.

http://www.slowmag.com/

Avoid products with magnesium oxide in them, or hydroxide, as they don't get absorbed by the GI tract and are eliminated in the stool. They are commonly used today as laxatives, and an example of this is Phillips Tablets. Some products show mixture of oxide and other chelates on the labels, avoid mixtures too. They are mostly oxide in reality.

You don't need high doses of magnesium, but you might need some to bring you up to RDA standards.

A good resource to check your food content is this:
SELF Nutrition Data | Food Facts, Information & Calorie Calculator

Beans are an excellent source of dietary magnesium. 3 oz of unsalted almonds have 270mg in them. Easy to do for a snack or added to salads.