Fill a glass with tap water, check the tds, and let it sit out for 24 hours. After that, check the tds again and see if anything dissipated overnight.
Makes sense, but I'm wondering if the amount of CalMag you're adding is the correct amount to accompany the regular nutes.
What kind of problems were you originally having? It's quite possible that the problems were caused by something else, and switching to RO didn't correct the issue. I've seen lots of threads from growers using hard water, and they have great looking plants. That's not to say that the plants wouldn't benefit from RO water, I'm just saying that your tap water may not have been the issue.