One of the potential symptoms is muscle cramping. If you get cramps in spite of being well hydrated, consider picking up some magnesium 'oil' (not really oil but a 'simple syrup' with magnesium chloride in place of the sugar - it feels a bit oily, hence the name), spray it on the skin over a cramp, rub it in, and if that works, you have a possible indication of your magnesium status. A solution of water and magnesium sulfide (Epsom Salts) does similar but is a bit less effective (but Epsom salts are really cheap).

Also: if you decide to supplement, don't bother with magnesium oxide, as it is absorbed poorly. Personally, I take magnesium citrate which is absorbed well and is inexpensive, but beware (!) it's also a laxative so build up slowly.

All that said, it's worth remembering that the MDRs are for avoiding acute deficiency disease. I subscribe to Bruce Ames' 'triage' theory of nutrition: nutrient needs to avoid acute disease get priority over those to avoid chronic conditions. Magnesium is a perfect case in point: you've laid out that "robbing Peter to pay Paul" process beautifully.