There are a variety of ways to test sodium chlorite and citric acid concentrations. Most of them involve adding reagents and then calculating the precise amount of a reaction chemical needed to provide the reaction.
You can get a general idea with a minimum of equipment if you have something known to go from.
If you have citric acid powder you can mix up a known concentration. Then you take a container of water, measure the pH of the water, then add a specific amount of your citric acid and finally measure the pH change in the water. This serves as your control. You then take the same amount of the citric acid in question and see if you end up with the same pH.
Sodium chlorite solutions can be checked with a Brix meter. Or you can go with the theoretical numbers and see how close you come.
For example a 28% by weight sodium chlorite solution will show 28 on the Brix meter. Since it is made from 80% pure powder your maximum yield is 224000 PPM. If you have a way to measure chlorine dioxide concentration you can mix some up and see what you get.
Another way to check is to mix up a typical batch and measure the pH and the concentration of chlorine dioxide in it. Take note of this and if you have questions about a new purchase you can run the test again and see if the numbers compare.