There are blood tests but apparently they're not very accurate because only a small % of magnesium is contained in the blood serum. If you're under a lot of stress, drink carbonated or caffeinated beverages, eat a lot of sweets, drink alcohol, or are anxious/hyperactive, most likely you're low of magnesium and could benefit from supplements. You could just try supplementing and see if it helps. It's very difficult to overdose from magnesium. You'll just the "runs" if you take too much - some people's stomachs can't handle too much (magnesium glycinate is much easier on the digestive system). Obviously if you take mass quantities it can affect your heart etc., but that's very rare and usually just found in people who have kidney problems. My doctor said I can take 1500mg a day and that it's pretty harmless. I find it very relaxing; my friend also found that it helped her anxiety.