If you're unsure whether your antifreeze is still good, there are a few ways to test it. One way is to simply look at the color of the fluid; if it's still bright green, it's probably still effective. Another way is to pour a small amount of antifreeze into a clear glass and see how long it takes to evaporate; if it takes more than 24 hours, the antifreeze is still good. Finally, you can use a hydrometer to test the freezing point of the antifreeze; if it's below -35 degrees Celsius, it's still good.