I bought a trace mix at a hydroponics store that is close to CSM+B. But there is no mg in the mix. every other mirco in this mix is at the same percentage as CSM+B.

I live in an area that has some of the hardest waters in the country (Canada) our GH here is around 40. Can I assume that my water has enough MG in it already?

I would say you are okay.

You can always try a bit and see if the plant COLOR changes, as I believe that this is one of the benefits, but not the only one.

Hope this helps.