Cooling system trouble usually starts small. The reservoir sinks a little between checks, a faint sweet smell appears after a drive, or a damp spot shows up on the driveway. Each of these is easy to ignore, and each is the kind of early sign that an engine failure later turns into a long bill.
What a slow leak looks like
Watch for a coolant level that falls between checks when you have not topped it off. Look for crusty residue around hoses, the radiator, the water pump, and the cap. Notice whether the heater blows cooler than it used to, or whether the temperature needle climbs in slow traffic and settles back on the open road. A puddle under the car, usually green, orange, or pink, is a clear sign to stop checking and start getting the car looked at.
Check the level the right way
Use the owner's manual to find the correct check procedure and the correct coolant type. Check only with the engine cold, and never open a hot cap. If the level is low, add only the type the manual names. Note the level, the day, and who checked it, and keep a photo of the reservoir mark. A plain written log is the strongest evidence you can bring to a claim.
Protect the claim as well as the engine
Many contracts ask you to show that you kept up with maintenance the manufacturer requires. If the manual calls for a coolant service at a set interval, keep the receipt for it. Also read the contract for any cooling system line and for how it treats a leak from a worn hose or gasket. Some contracts treat those as wear. Others treat them as a failure. Ask which, and get the answer in writing.
When to stop driving
If the temperature warning light comes on, pull over somewhere safe, switch off the engine, and let it cool. Do not keep driving to reach a shop. A few minutes of extra driving can turn a hose repair into an engine repair, and that changes what the contract is asked to pay for.
For the posted terms on sample contracts, which show how a provider words its wear and failure lines, start with the providers that publish a sample contract page. Read the wording before you need the claim.