Anti-freeze mixture can be checked with a standard
bulb type tester.
You are looking for 50/50 for proper cooling. Stronger and you get poor heat transfer.
A 50/50 mix will give you a reading of about -34 F. [-37C].
Check cold. If you are in warmer climates, a 70/30, water/AF is fine and will gives much better heat transfer.
Anything over 50/50 drastically reduces cooling and is
a common mistake made after doing cooling system repairs.
Best to pre-mix 50/50 whenever adding coolant.
Then you know where you are.