How to Know When You Need to Change or Flush Your Coolant. Do you need new antifreeze or a radiator flush? Learn 4 different methods to test your coolant including doing a visual inspection, using a hydrometer to test the freezing point and boiling point, using a multimeter to check the antifreeze, and testing pH. After watching this video you will know if your coolant is still good or if you need to do a coolant flush and fill! The multimeter I used (under $20 here): http://www.amazon.com/gp/product/