If both products are already mixed it's unlikely that you're going to find out their potency beyond whatever information you might have been given on matters.
That being said, testing the solutions should be relatively easy based on activation. Though as you yourself already mentioned, either your acid may have weakened OR the Miracle-Mineral-Supplement solution may have begun to degrade. At which point, you might save it by using it to make CDS instead(which is very good for topical applications etc)
PS. you might look into making a stronger mixture of citric acid or even HCl if you have access to it, to test and see if you can get a better activation with the Miracle-Mineral-Supplement .
PS. I've had Miracle-Mineral-Supplement in the cupboard for over 3 years and it activated just fine with HCl. I used it to make CDS rather than throwing it out.