It doesn’t seem to be a limiting element like others in the hobby. Typical recommendation is to keep in the ballpark of 1300-1500. With such recommendations any of the colorimetric kits like Salifert, Red Sea, etc will get you in the ballpark.
I do have a trident and found that my tank went through stages where magnesium consumption seemed to increase for a short period of time and then would go back to being almost negligible. At any rate, many successful reefers don’t test for it at all.