Once you have an adequate sample of coolant in the test tube, hold it up to any light source so that you can see through the tube of coolant. The sky works best if you can be outside. Give it 30 seconds or so to stabilize, and then look to see how many balls have floated to the top of the tube. The more balls float to the top, the more protection against freezing your car has. There will be markings on the tube to use as a guide, and they are usually explained further on the package.