What you need to do is buy some test strips. Most stores don't stock them anymore, but can get them for you in a day or two. NAPA and Carquest are 2 places that's usually easy to order them from. The more popular chains just stare at you dumbfounded. Also fleet shops or farm tractor dealers might have them.

Don't buy any fancy kit, get the cheapest one they have. It will be a bottle or package with little strips of paper inside. It will come with a color chart and instructions. Basically dip a strip of paper into your coolant and match it up to the color chart per the directions. If needed, add your bottle of SCA, drive around a while and re-test. Repeat this process until the SCA levels look good on the test strip.