What is the correct way to check your car coolant?

Check the level in the expansion tank/reservoir.  Most, if not all, vehicles both a HOT and COLD level on the bottle that shows you the proper level.

If you must add coolant, only open the cap while the engine is cold.

Wait until the car is cold, open the hood, and look at it.  You can see the reservoir just by looking at it, it should be at or near the full or max line.  Then take off your radiator cap, you should see the coolant right there.

wait till its cold ..the engine ..your car will have an expansion tank ..next to rad ..there are min and max marks on side of expansion tank ..fill to max ONLY  ..the max mark may be only in middle of tank ...its not called an expansion tank for nothing 

Read the owner's manual.