To check your coolant level, you simply need to look at the white plastic overflow tank beside your radiator. These containers usually have high-low level labels on them which will help you determine if it’s time for you to replenish you coolant. If your coolant level is already low, just remove the cap on your coolant tank and add your mixture until it reaches its recommended level again. And then, place back the cap to close the receptacle.
Replenishing your coolant without determining the reason for loss is imbecilic.