I don't take any supplements at the moment. If you eat sea food I wouldn't be too worried about iodine, but you can get that tested if you are. Depending on where you are in the world, you might not be getting enough of it.

As for magnesium, due to the nutrient depletion of our soils, the animals we eat might not be getting enough magnesium from the plants they eat. I usually recommend magnesium citrate or magnesium glycinate because of their better absorbtion. The glycinate form can cause drowsiness and is best taken at night, the citrate form can cause loose stools. You can test magnesium in blood but it's not very reliable since this is a predominantly intracellular mineral, the recommended test is a magnesium red blood cell test.

As a carnivore, I would simply experiment and test only if I have concerns or am curious.