There is a tool that pros use to check coolant concentration and it is the only one I trusted as freeze protection was very important where I was doing most of my coolant work. It is is prizm (spygometer? not sure on spelling) type device. You put one drop of coolant on the lens and look through it and a mark appears on a scale telling you how cold you are safe for.
I never felt the floating ball type tools were accurate enough but then I had to put my signature on how good the coolant is and if I have to sign for something you can bet I will make sure what I sign is accurate.