I need help with this plumbing questiion. I have a natural gas boiler, the plumber told me I need new?

anti freeze installed in the lines. The cost to do this would be 700.00. I bought a automotive anti freeze tester ,and checked, it said it was good for -40 degrees. I called the plumber and he said you must check it by putting a sample in the freezer compartment of your refrigerator over nite and see if the sample freezes, what is the correct way to check this? thanks for the help