THrough a blood test and or urine test.
I never needed to do one, I just kept loosing my sodium and ending up in the hospital this is when I was tested for Addison's disease and dignosed with it. DR's knew then I had low aldesterone how my sodium reading were from blood work.
I just recently stopped loosing my sodium I am with no support such as meds or even salt. I rid my body of huge amonuts of aluminum in NOv/Dec 2012. Since then no issues with sodium loss. But I do have to becareful with to much potassium supplements. I can now eat foods high in potassium now, which I was not able to do before.